Long before the reader has come to this part of my book, a multitude of difficulties may have presented themselves to him. Some of them are of such weight that I cannot think of them without faltering; but to the best of my knowledge most of them are only apparent, and those which are in truth should not be pernicious to my theory.
These difficulties and objections can be summed up in the following headings: First, if species have arisen from other species through imperceptibly small gradations, why do we not see innumerable transitional forms everywhere? Why doesn't all of nature offer a mishmash of forms instead of the well-defined species?
Secondly, is it possible that an animal e.g. B. with the organization and way of life of a bat, through the transformation of some other animal with a completely different way of life? Is it believable that natural breeding on the one hand[p. 198]Organs of such insignificant essence as e.g. B. the tail of a giraffe, which serves as a fly whisk, and on the other hand produce organs of such wonderful structure as the eye, the inimitable perfection of which we scarcely fully comprehend.
Third, can instincts be acquired and modified through natural breeding? What should we Say, for example, of so wonderful an instinct as is that which causes the bee to form cells, by which the discoveries of profound mathematicians are practically anticipated.
Fourth, how is it to be understood that species when crossed are sterile or give sterile offspring while the fertility of crossed varieties remains undiminished.
The first two of these principal questions will be discussed here, and the last, viz., instinct and hybrid formation, in separate chapters.
Lack or rarity of mediating varieties. ) Since natural selection works only through the preservation of useful variations, every new form in an already fully populated region will strive to replace and ultimately exterminate its own lesser perfected elders as well as all other lesser perfected forms with which it comes into competition. Natural selection, as we have seen, goes hand in hand with this activity. If, therefore, we regard each species as descended from some other unknown form, the ancestral and transitional forms will usually have already been exterminated by the process of formation and perfection of the new form.
But if, according to this theory, innumerable transitional forms must have existed, why do we not find them embedded in infinite numbers in the strata of the earth's crust? It will be more appropriate to discuss this question in the chapter on the incompleteness of the geological record. Here I will only say that I think the answer lies mainly in the fact that those documents are incomparably inferior[p. 199]are more complete than is usually supposed, and that that incompleteness of the geological record is chiefly due to the fact that organic beings do not inhabit very great depths of the sea, hence their remains could only be enclosed and preserved for future times by such masses of sediment as are sufficient been thick and stretched out to avoid an immense measure of subsequent destruction. And such fossil-bearing masses can accumulate only where much precipitation is deposited in shallow seas during slow subsidence of the ground. These coincidences will occur only rarely and only after extraordinarily long intervals. While the sea floor is at rest or in the process of being raised or only light precipitation is taking place, the pages of our geological history books remain blank. The Erd-rinde is a vast museum, but its natural history collections have only been introduced in individual periods that are infinitely far apart.
It may be objected that if some closely related species now live together in the same region, many intermediate forms must certainly be found. Let's take a simple case. Traversing a continent from north to south, one usually meets with other closely allied or representative species from time to time, apparently occupying about the same place in the natural economy of the country. These representative species are often contiguous or encroaching on each other's territories, and as some become rarer and rarer, the others become more and more common until they replace one another. If we compare these species where they mingle, they are usually just as completely different from each other in all parts of their structure, than like the specimens taken from the center of the distribution district of each. Now, according to my theory, all of these species have a common progenitor and each of them is only adapted to the living conditions of its area through the modification process[p. 200]has replaced and suppressed its original stock as well as all intermediate stages between its first and present form, so that we can no longer expect to find numerous transitional forms in every region, although they must have existed and their remains probably too may have been absorbed into the layers of earth. But why do we not also find closely related transitional varieties in the intermediate regions, where the external conditions of life form a transition from those of one region to those of the other? This difficulty has puzzled me for a long time; but I now think it can be explained in large part.
Above all, we should be very careful in assuming that because an area is now continuous, it has been continuous for a long time. Geology leads us to believe that almost every continent was divided into many islands as late as the Tertiary period; and on such islands distinct species may have formed separately, with no possibility of furnishing intermediate forms in the intermediate regions. In consequence of the changes in land-form and climate, the now contiguous sea-regions may have been less contiguous and uniform at a comparatively late period. But I will refrain from this means of escaping the difficulty; for I believe that many quite distinct species arose in quite contiguous areas,
With regard to the present distribution of the species over wide areas, we find that they usually occur in fairly large numbers over a large part of it, but then become more and more rare towards the frontiers, and finally disappear altogether; hence the neutral area between two representative species is usually narrow in proportion[p. 201]to that which each of them peculiarly inhabits. We make the same remark when we climb mountains, and it is sometimes very striking how suddenly, according to A LPHONSE DE CANDOLLE 'S observation , a common species disappears in the Alps. E DW. FORBES _made the same perception when he trawled up the denizens of the lake bottom. This fact must perplex all those who regard the external conditions of life, such as climate and altitude, as the almighty causes of the dispersal of the forms of organisms, the change of climate and altitude or depth being everywhere gradual. But if we remember that almost every species, even in the midst of its homeland, would increase to immeasurable numbers if it were not in competition with other species—that almost all live on other species or serve as food for them—in short, that every organic being has a direct or indirect relationship to other organisms, we must recognize that with which they compete; and as these species already have a definite limit, and are no longer imperceptibly merging, the range of a species dependent on one or the other must be sharply defined. Moreover, every species, at the limits of its range, where its numbers are in any case diminishing, must very often be exposed to utter destruction by fluctuations in the numbers of its enemies or its prey, or in the seasons, and this may also result in the sharper definition of its geographic distribution with be conditional.
If my opinion is correct, that allied or representative species inhabiting a contiguous area are usually so distributed that each of them occupies a wide range, and that these ranges are proportionally distributed[p. 202]narrow neutral intervals are separated, in which each species rapidly diminishes in abundance—then, as varieties do not differ essentially from species, this rule will probably apply to both; and if we mentally adapt a variable species to a very large area, we shall have to adapt two varieties to those two large sub-areas, and a third variety to the narrow intermediate area. This intermediate variety, from occupying a smaller space, will be in smaller numbers; and in fact, as far as I can ascertain, this rule applies quite well to varieties in the state of nature. I have found good evidence of this rule in varieties of Balanus species intermediate between more marked varieties of the same.Thanks to ATSON , A SA G RAY , and W OLLASTON , that usually when middle varieties occur between two other forms, they far outnumber those which they combine. Now if we accept these facts and evidence as correct, and conclude that varieties which unite two other varieties were usually present in fewer numbers than these latter, it might also be understood why intermediate varieties do not last long and, as a general rule, must be exterminated and extinct sooner than those forms which originally concatenate them.
For a less numerous form will, as before remarked, be altogether more in danger of being exterminated than those in abundance; and in this particular case the intermediate form might be preferentially subjected to the attacks of the two closely allied forms on either side of it. But a far more important consideration seems to me to be that during the process of further transformation, whereby, according to my theory, two varieties are raised to two quite different species, these two varieties, insofar as they inhabit larger areas, also[p. 203] are present in greater numbers, and are therefore of great advantage over the medium variety, which in smaller numbers inhabits a narrow space. For forms which exist in greater numbers always have a better chance than the fewer ones of presenting other useful variations to Natural Selection within a given period. Therefore, in the struggle for existence, the commoner forms will strive to supplant and replace the rarer ones, which are only able to change and perfect themselves slowly. It seems to me to be the same principle, according to which, as shown in the second chapter, the common species of a region also present on average a larger number of varieties than the rarer ones. I will now, in order to explain my opinion better, assume there are three Schaaf varieties, one of which is suited to a vast mountainous region, the second only to a comparatively narrow strip of hills, and the third to broad plains at their foot; I will also assume that the inhabitants are all striving with equal skill and zeal to improve their breeds by breeding, in which case the probability of success will be entirely in favor of the large herd owners in the mountains and in the plains, because these improve their races quicker than the small ones in the narrow hilly intermediate zone, so that the improved mountain or plain race will soon take the place of the less improved hill country race; and so the two races, which at first existed in great numbers,
In sum: I believe that species can be reasonably well defined without at any time presenting an unraveled chaos of variable and intermediate forms: 1) because new varieties are very slow to form, variation being an extremely sluggish process and natural breeding so can do nothing for a long time unless favorable deviations occur and not a place in the natural balance of the area[p. 204]can be filled out better by modifying one or the other of its inhabitants. And such new places will depend on slow changes in climate, or on the accidental immigration of new inhabitants, and, probably to a much greater degree, on some of the old inhabitants being slowly modified, while those newly originated and immigrated forms with some old ones in Interaction advised: hence we get to see but few species in any region and at any time, which present little tolerably permanent modifications of organization. And we certainly see it that way.
Secondly, many now contiguous areas of the earth's surface must have been divided into distinct parts as late as the present epoch, in which many forms, especially mating and migratory animals quite distinct from one another, have been able to differentiate sufficiently to count as species be able. Intermediate varieties between these species and their common ancestral form must have formerly existed in each of these fractions of the range, but have afterwards been replaced by natural selection and exterminated, so that they are no longer alive.
Thirdly, if two or more varieties have been established in the different parts of a contiguous district, intermediate varieties will probably also have arisen in the narrow intermediate zones, but have not lasted long. For these intermediate varieties, for reasons already established (and particularly what we know of the present distribution of closely related species and mature varieties), are found in fewer numbers in the intermediate zones than the principal varieties which they unite in their own districts. For this reason alone the intermediate varieties will be subject to occasional extinction, but will certainly be supplanted and replaced during the process of further modification by the forms which concatenate them, mostly because[p. 205]easier to further improve and thereby secure through natural breeding.
Finally, if my theory is correct, there must certainly have existed, not just at one time but at all times, innumerable intermediate varieties connecting the species of the same group with one another, but also the very process of natural selection must have been constantly at work, both to annihilate their stem-forms as the middle links themselves. Therefore, proof of their earlier existence could at most be found among the fossil remains of the earth's crust, which, however, as will be shown in a later section, is only suitable for preserving these documents of earlier times in a very imperfect and incoherent way.
Origin and transition of organisms with a peculiar way of life and organization.) Opponents of my views have asked me how e.g. For example, could a land beast of prey be transformed into a water beast of prey, since an animal in an intermediate state could not well exist? It would be easy to show that within the same group of beasts of prey there are animals which occupy every intermediate stage between simple land and true aquatic animals and are therefore well suited by their different ways of life to fight with others for their existence place to claim. So e.g. For example, the North American Mustela vison has webbed toes and resembles the otter in fur, short legs, and the shape of the tail. Throughout the summer this animal dives into the water and feeds on fish; but during the long winter it leaves the frozen waters and, like other polecats, feeds on mice and land animals. Had you chosen a different case and asked me how an insectivorous quadruped was transformed into a flying bat, that question would be far more difficult to answer. But in my opinion such individual difficulties do not carry too great a weight.
Here, as in other cases, I find myself in the great[p. 206]Disadvantage of being able to give only one or two instances, out of the much pertinent evidence which I have collected, of a transition in habits and organization between closely allied species in a genea, and of temporarily or permanently altered habits in the same species. And it seems to me myself that nothing less than a long list of such instances would suffice to remove the difficulty of explaining a case so singular as that of the bat.
If we look around in the family of the squirrels, we find there the first weak transition stage to the so-called. flying bat-mice indicated in the two-line flattened tail of one and, according to J. R ICHARDSON ' Sremark in the broadened hindquarters and fuller skin on the sides of the body of the other species; for in flying squirrels the hind limbs, and even the beginning of the tail, are united by a considerable extension of skin, which serves as a parachute, enabling these animals to soar through the air to astonishing distances from tree to tree. There is no doubt that every part of this peculiar organization is useful to every species of squirrel in its homeland, enabling it to escape the pursuit of birds of prey or other beasts of prey, to gather more plentiful food, and no doubt to lessen the danger of each falling . However, it does not follow from this that the organization of each squirrel is also the best possible for all natural conditions. Assuming that climate and vegetation change, new rodents appear as competitors, and new predators immigrate, or old ones undergo modification, we should also, by all analogy, surmise that at least some of the squirrels will diminish in number or become extinct altogether, if their organization is not also modified and improved in a corresponding manner. Hence I find no difficulty in assuming, especially when the external conditions of life change, that individuals with ever fuller lateral skin preferentially thus, by all analogy, we ought to surmise that at least some of the squirrels will diminish in number or become extinct unless their organization is also modified and improved in a corresponding manner. Hence I find no difficulty in assuming, especially when the external conditions of life change, that individuals with ever fuller lateral skin preferentially thus, by all analogy, we ought to surmise that at least some of the squirrels will diminish in number or become extinct unless their organization is also modified and improved in a corresponding manner. Hence I find no difficulty in assuming, especially when the external conditions of life change, that individuals with ever fuller lateral skin preferentially[p. 207]should be preserved, because this character is hereditary and every strengthening of it is useful until, through the accumulation of all the individual effects of this process of natural breeding, the squirrel has finally become a flying squirrel.
Let us now consider the flying lemur, or Galeopithecus, which has heretofore been erroneously classified as bats. It has a very broad lateral skin, extending from the posterior ends of the jaws to the tail, enclosing the legs and extended fingers, also provided with an extensor muscle. Although there are now no intermediate intermediate stages between the common Lemurids and the air-sliding Galeopithecus, I see no difficulty in believing that such intermediate stages once existed and evolved from stage to stage in a manner similar to that of the intermediates above the squirrels and flying squirrels, each further step towards improving the organization in this direction being beneficial to the owner. Nor can I see any insurmountable difficulty in further assuming that both the forearm and the fingers of Galeopithecus, which are connected by the wing skin, have gradually lengthened as a result of natural breeding, and this would suffice to transform them into a bat as far as the flying tools are concerned. In those bats whose wing skin extends only from the shoulder to the tail, including the hind legs, we may still see traces of a device which was originally made more for gliding through the air than for flying. to turn into a bat. In those bats whose wing skin extends only from the shoulder to the tail, including the hind legs, we may still see traces of a device which was originally made more for gliding through the air than for flying. to turn into a bat. In those bats whose wing skin extends only from the shoulder to the tail, including the hind legs, we may still see traces of a device which was originally made more for gliding through the air than for flying.
If about a dozen peculiarly formed families of birds had become extinct or remained unknown to us, how could we have dared to surmise that there ever existed birds which, like the stubborn duck (Micropterus Eyton 's ) , only had their wings like flaps to flutter over the surface of the water, or like the fat geese used like oars in the sea and like forelegs on land, or like the ostrich like sea slings to move the barrel, or finally, like the Apteryx, didn't use them at all. And yet[p. 208]the organization of each of these birds, under the conditions of life in which it finds itself and struggles for existence, is beneficial to it, though not necessarily the best of all possible arrangements. Incidentally, it should not be concluded from these remarks that any of the gradations of wing formations just mentioned, all of which are perhaps only the result of disuse, belong to a natural series of stages on which the birds ascending have acquired the perfect power of flight; but they can at least serve to show what various ways of transition are possible.
When it is seen that a small number of animals from the water-breathing classes of the crustaceans and mollusks are adapted to life on land, when it is seen that there are flying birds, flying mammals, flying insects of the most varied types, and formerly flying If reptiles existed, it is also understandable that flying fish, which now with the help of their flapping pectoral fins rise obliquely over the sea level and glide in a wide arc through the air, can gradually be transformed into perfectly winged animals. And once this was done, who would ever imagine that in an earlier time they were inhabitants of the open sea and, as we now know, only used their incipient flight organs to escape the jaws of other fish.
Whenever we see an organ highly developed for any particular purpose, such as the wings of a bird for flight, we must remember that such animals, being at the initial stage of this development, have seldom had a chance of surviving to our day , precisely because they must have been continually replaced by other more advanced forms through the perfecting process of natural selection. We shall further consider that transitional stages between formations serving quite different ways of life may have seldom been developed in great numbers and with various subordinate forms in earlier times. But, to return to our flying fish,[p. 209]thus it does not seem very credible that fishes capable of actual flight should have been developed in many subordinate forms for catching various prey by various routes, on water and on land, until they attained a decided superiority over other animals in the struggle for existence. Hence the probability of discovering species still in the fossil state at transitional stages of organization will always be small, because they have existed in fewer numbers than species with fully developed structures.
I will now give two or three instances of modified and divergent habits in individuals of the same species. When the case arises, it will be easy for natural selection, by some modification of its structure, to make an animal fit for its altered habits of life, or for just one of its various habits. But it is difficult to say, and immaterial to us, whether habits generally change first, and then organization, or whether slight modifications of structure lead to a change in habits; probably both change at the same time. As to change of habits, that would suffice for the multitude of Britishto refer to species of insects, which now live on foreign plants, or entirely exclusively on artifacts. Countless examples could be given of the divergence of habits. I have often observed a species of South American shrike (Saurophagus sulphuratus) hovering like a kestrel over one spot and then over another, and at other times standing stiffly at the water's edge and then suddenly pouncing like a kingfisher on a fish. In our own region one sees the great tit (Parus major) sometimes climbing around the branches almost like a treecreeper, sometimes killing small birds by blows on the head like a shrike; I have often seen her hammer open the seeds of the yew tree on a branch, and then break them open like a nut chopper. InNorth America swims up four hours after H EARNE 'S observation of the black bear[p. 210]long with wide open mouth in the water to catch water insects almost like whales [25] .
As we sometimes see individuals following habits which differ greatly from those of other individuals of their kind and of other species of their kindred, we should, on my theory, have expected that such individuals would sometimes give rise to new species, with differing morals and a more or less modified organization give cause. And such cases occur in nature. Can there be a more apt example of adaptation than the woodpecker, which clambers up trees to seek out insects in cracks in the bark? Yet in North America there are woodpeckers, which live chiefly on fruit, and others with elongated wings, which snatch insects on the fly; and on the plains of La Plata, where no tree grows, there is a woodpecker (Colaptes campestris), which has two toes in front and two behind, a long pointed tongue, stiff tail-feathers, and a straight powerful beak. However, the tail feathers are less stiff than in the more typical species, and serve to help the bird descend perpendicularly to the ground. The beak is also less straight, although strong enough to hack into wood. Another illustration of the varied habits of this group of birds is afforded only by a Mexican colaptes, which, according to DE S AUSSURE , carves holes in hard wood to store a store of acorns for its future consumption. Accordingly, the woodpecker is from La Plataa true woodpecker in all essential parts of its organization and has also until recently been accommodated in the typical clan. Even such insignificant characters as its coloring, the shrill tone of its voice, and its undulating flight, all convinced me of its close blood affinity with our common woodpeckers. But this woodpecker, as the exact A ZARA already assures, never climbs trees.
[p. 211]
Of all birds, petrels are those which fly best and are most attached to the high seas; and yet in the calm still straits of Tierra del Fuego there is a species, Puffinuria Berardi, which, by its habits in general, by its amazing ability to dive, by its manner of swimming and by flight, when compelled to fly against its will, would be thought by everyone to be an auk or a grebe (Colymbus); but in essence it is a petrel, with only some profound changes of organization; while at the Woodpecker of La Plata the body structure has undergone only insignificant changes. In the case of the dipper (Cinclus), on the other hand, one would not be able to discover the slightest trace of its water-bound way of life, even with the most exact examination of the body. And yet this so deviant member of the thrush family earns all its subsistence by diving, by scraping up the debris with its feet, and by using its wings underwater.
Whoever believes that every creature was created as we now see it must have sometimes been surprised to find an animal whose organization and habits were not at all in harmony. What can be plainer than that the feet of ducks and geese, with the great skin between the toes, were made for swimming? and yet there are geese with such webbed feet, which seldom or never go into the water; — and except for A UDUBONno one has ever seen the frigate bird, whose four toes are webbed, alight on the surface of the sea. On the other hand, grebes and coots are excellent waterfowl, and yet their toes are but webbed. What seems clearer than that the long toes of marsh-birds are given them to enable them to stride over marsh-bottom and floating aquatic plants, and yet the reed-fowl (Ortygometra) is almost as much a water-bird as the coot, and the rail almost as much a land-bird as these[p. 212]quail or the partridge. The webbed foot may be said to be atrophied in function, but not in form. In the case of the frigate bird, on the other hand, the deep section of the webbing between the toes shows that a change in foot formation has begun.
Those who believe in countless separate acts of creation will say that in these cases it pleased the Creator to designate a being of one type for the place of a being of the other type. But this seems to me the same thing again, only in a more dignified version. Anyone who believes in the struggle for existence and in the principle of natural selection will recognize that every organic being constantly strives to increase its number, and that, however little it varies in organization or habits, it thereby gains an advantage over anything else obtains another inhabitant of the region whose position it may take, however different it may be from its own previous position. He will not, therefore, be astonished to see web-footed geese and frigate-birds, by which some live on dry land and others only seldom set foot on the water, or to find long-toed reed hens (crex) living in meadows instead of swamps; or that there are woodpeckers where there are no trees, that thrushes dive under water, and petrels live like auks.
Organs of extreme perfection and composition. ) The assumption that even the eye, with all its unattainable devices for adjusting focus to the most varied distances, admitting different amounts of light, and improving spherical and chromatic aberration, has become what it is only through natural breeding , seems, I will frankly admit, to be absurd in the highest possible degree. When it was first pronounced that the sun stood still and the earth revolved on its axis, the common sense of man declared this doctrine false; but[p. 213]the old adage “vox populi, vox dei” has no validity in science. And yet reason tells me that when there are numerous gradations from a perfect and compound eye to a very simple and imperfect eye, all useful to their owner—when the eye is apt to vary somewhat, and its variations are hereditary, which is certainly the case--if a more or less considerable modification of an organ is always useful to an animal whose external conditions of life are changing: then the supposition that a perfect and compound eye could be formed by natural selection seems certainly no longer face any serious difficulty, however difficult the conception of it may be for our imagination. The question, how a nerve becomes sensitive to light scarcely troubles us more than how life itself originates. However, I will remark that various facts lead me to suspect that every sensitive nerve may be sensitized to light, and also to those grosser vibrations of air which produce sound.
As to the gradations by which an organ has been perfected in any species, it is true that we should only seek them out in a direct line in their ancestors. But this is hardly ever possible, and we are in any case obliged to choose among the species of the same group, i. H. among the collateral relatives of the same descent with the first, to see what gradations are possible, and whether it is probable that any gradations from the first progenitors onwards have been transmitted to the present descendants with little or no modification be. Among the extant vertebrates we find little gradation in the formation of the eye (although the fish Amphioxus has a very simple eye without a lens), and no further investigation can be made of it in fossil beings. We would probably have to go way back before the lowest fossiliferous strata[p. 214]to discover the first stages of perfection of the eye in this circle of the animal kingdom.
In the lower kingdoms of the insects one can proceed from an optic nerve simply coated with pigment, which often forms a kind of pupil, but is without a crystal lens or other optical apparatus. From these rudiments of the eye, which is able to distinguish light from darkness, but nothing else, perfection progresses in two directions, which J. M ÜLLERfundamentally different believes; namely, it leads either 1) to stemmata or so-called. “simple eyes” provided with a crystal lens and cornea, or 2) to “compound eyes” which act alone or chiefly by excluding all rays coming from any point of the object seen, except for that bundle of rays which falls perpendicular to the convex retina. These compound eyes, now with endless differences in form, proportion, number, and position of the transparent cones coated with pigment, which act only by exclusion, are soon joined by a more or less perfect concentrating device, in that in the eyes of Meloe, e.g . For example, the facets of the cornea become somewhat convex on the outside and inside, and thus become lens-shaped.ILNE E DWARDS "renflemens lenticulaires paraissent s'être développés", and sometimes these lenses detach from the cornea as a separate layer. The transparent pigment-coated cones, which Muller believed to act only by shutting out divergent pencils of light, are usually attached to the cornea, but are not infrequently detached from it, and present a convex outer surface; in my opinion they must act like converging lenses in this case. The structure of the compound eyes is so varied that MÜLLER assumes three main classes with no less than seven subdivisions according to their structure. He forms a fourth[p. 215]Principal class from the "clustered eyes" or groups of stemmata which, according to his explanation, constitute the transition from the mosaic-like "compound eyes" without a device of concentration to the organs of the face with such a device.
Considering these facts, here all too briefly and imperfectly indicated, showing that there are many gradual differences in the formation of eyes even among the insects now living, and considering how small the number of living species is compared with those already extinct, thus I can find (though more so than in other formations) no too great difficulty in supposing that the simple apparatus of an optic nerve surrounded by pigment and covered by a transparent membrane has been transformed by natural selection into so perfect an optical instrument as it found in the most perfect insects.
Anyone who wants to go further, if he finds while reading through this book that a large number of otherwise inexplicable facts can be understood through the theory of descent, need have no scruples about the further assumption that through natural selection finally such a perfect Structures than the eagle's eye can be produced, even if the intermediate stages in this case are completely unknown to him. His mind must overcome his power of imagination. Yet I myself have felt the difficulty far too well to wonder in any measure that any one should dare to stretch the theory of natural selection to so amazing an extent.
One can hardly avoid comparing the eye to a telescope. We know that this tool has been improved by the long-continued efforts of the highest human intelligence, and naturally conclude that the eye attained its perfection through a somewhat similar process. But shouldn't this idea only be imaginary? Do we have a right to suppose that the Creator works by virtue of intellectual powers similar to those of[p. 216]People? If we were to compare the eye to an optical instrument, we would have to imagine a thick layer of transparent tissue, soaked with liquid, and with a nerve sensitive to light beneath it, and then suppose that each part of this layer slowly but steadily changed in density, so that different layers of different densities are superimposed and at unequal distances from each other, and also that the surface of each layer slowly changes shape. We must further suppose that there is a force (Natural Selection) which is constantly observing every slight accidental change in the transparent layers and carefully selecting every variation, which, under changed circumstances, would in some way or in some degree be apt to produce a clearer picture. We would have to assume that each new condition of the instrument is multiplied by a million, and each one is preserved until a better one is produced, but then destroyed. In living bodies, variation produces those slight differences, generation multiplies them to infinity, and natural selection, with unfailing tact, discovers every improvement for the purpose of further perfection. If one now imagines this process for millions and millions of years and continued every year with millions of individuals of the most diverse kind: should one not expect that the living optical instrument must finally become more perfect than the glass one to the same degree,
If any compound organ could be found which could not be brought to completion by innumerable small successive modifications, my theory was bound to collapse. However, I cannot find any such case. Doubtless there are many organs whose degrees of perfection we do not know, especially in very isolated species, whose related forms, according to my theory, have largely become extinct. So must also where[p. 217]it is an organ common to all members of a sub-kingdom, this organ must have been formed in a very early prehistoric period, because all members of this sub-kingdom only developed afterwards; and if we wanted to discover the earliest transitional stages that the organ had to go through, we would have to look around at the earliest beginning forms, which have long since died out.
We must be careful in asserting that an organ could not have been formed by gradual changes of any kind. One could cite numerous instances of how in the lower animals one and the same organ performs quite different functions: the alimentary canal in the larva of the dragon-fly, however, breathes and digests and excises, as in the fish Cobitis. If the Hydra is turned inside out like a glove, the outer surface digests and the inner breathes. In such cases, by natural selection, a part or organ which hitherto had two functions could easily have been developed exclusively for one of the two purposes, and the whole nature of the animal gradually changed, if this had been useful for it from the beginning would. certain plants, such as some legumes, violacea and others. produce two kinds of flowers, one with the formation due to its order, the other stunted, but sometimes more fertile than the first. If such a plant refrained from producing flowers of the first kind for several years, as is the case in a If a specimen of Aspicarpa imported from France were actually done, there would indeed be a great and sudden change in the nature of the plant . Two different organs sometimes perform the same service together in the same individual, as e.g. B. Fishes are endowed with gills, by which they breathe in the air dispersed in the water, while at the same time being able to breathe atmospheric air with their swim-bladders, which are connected at the end by an air-duct with the fauces, and are internally very vascular dividing walls (Lepidosiren). In this case it can[p. 218]It is easy for one of the two organs to be altered and so perfected that it more and more does all the work on its own, while the other either passes over to a new purpose or withers away altogether.
This example of the fishes' swim-bladder is very instructive, because it shows us the all-important fact how an organ originally formed for a particular purpose, viz., swimming, may be modified for an entirely different function, namely, respiration. Also, the swim-bladder has been processed as an ancillary part of the auditory organ of some fishes, or (I do not know which interpretation is now most commonly accepted) a part of the auditory organ has been used to complement the swim-bladder. All physiologists agree that the swim-bladder is "homologous" or "ideally similar" in position and structure to the lungs of higher vertebrates; hence the assumption that natural breeding has transformed a swim-bladder into a lung, or purely respiratory organ,
In fact, I can scarcely doubt that all vertebrata with true lungs descended by the ordinary reproductive route from some ancient unknown ancestor with a swimming apparatus or a swim-bladder. That's how you like yourself, like me from Professor O WEN 'S interesting description of these parts, explaining the curious fact of how it is that every particle of food and drink which we take must slip off the mouth of the trachea with some danger of falling into the lungs, the ingenious arrangement notwithstanding, which closes the epiglottis. In the higher Vertebrates the gills have entirely disappeared, but the clefts on the sides of the neck and the looping course of the arteries seem to indicate their former place in the embryo of man. Yet it would have been understandable if the gills, now entirely gone, had been reworked by natural selection for quite another purpose; as according to some naturalists,[p. 219]that the gills and dorsal scales of certain annelids are homologous with the wings and elytra of the hexapod insects, it would be probable that organs which in very ancient times served for respiration were now transformed into organs of flight.
As for the passage of the organs to other functions, it is so important to become acquainted with the possibility of this that I will give one more example. The stalked barnacles (Cirripedes) have two little folds of skin, which I call egg-reins, which are designed, by means of a sticky secretion, to retain the eggs while they are hatching in the ovary. These barnacles have no gills, the whole surface of the body and sac, including the small reins, being used for breathing. The Balanids, or sessile Cirripeds, on the other hand, have no such reins, the eggs lying loosely at the bottom of the sac in the well-closed shell; but they have in the same relative position large puckered membranes,WHOMand have been declared to be gills to all other anatomists familiar with the subject. Now I think no one will dispute that the egg-tails of one family are homologous with the gills of the other, as indeed they are gradual. Hence I do not doubt that small folds of skin, which here at first served as reins for the eggs, and assisted in a small degree in respiration, have been gradually transformed by natural selection into gills, merely by increasing their size, and at the same time atrophying of the glands attached to them. If all stalked cirripedes had died out (and they have already experienced more extinction than the sessile ones): how could we ever have imagined that the respiratory organs of the balanids would originally have had the purpose
Although I have cautioned that one should be careful in assuming that an organ cannot possibly be formed by very gradual changes[p. 220]Although transitions may have been formed, I willingly admit that very difficult cases may occur, some of which I intend to discuss in my larger work.
One of the most difficult is formed by the asexual insects, which are often formed very differently from both the males and the fertile females of their species, but to which case I shall return in the next chapter. The electrical organs of fish present another case of peculiar difficulty; it is incomprehensible by what gradations the formation of these wondrous organs may have been effected. However, according to R. O WEN ' S and others, their innermost structure is quite similar to that of ordinary muscles, and since it has recently been shown that rays have an organ quite analogous to the electrical apparatus, but according to M ATTEUCCI ' Sinsurance discharge no electricity, we must confess that we are far too ignorant to pretend that no transition of any kind is possible.
But the electrical organs present other very serious difficulties. When one and the same organ appears in different members of a class, and especially with very divergent habits, its presence in these members may be inherited from a common progenitor and his Explain absence in others by loss through disuse or natural selection. But if the electric organ had been inherited by those fishes from an old predecessor who had been provided with it, then we might expect that all fishes that are still electric would also be closely related to one another in other respects. But paleontology gives absolutely no reason to believe that formerly most fish were provided with electrical organs, which almost all their descendants have lost. The presence of luminous organs in a few insects of the most diverse families and orders presents a concomitant difficult case. For example, in the vegetable kingdom there is the peculiar development of a mass of pollen grains[p. 221]a peduncle with a viscid gland at the end of which is quite the same in Orchis and in Asclepias, two genera possibly widely separated among flowering plants. Yet it may be remarked that in such cases, where two very different species are provided with what appears to be the same anomalous organ, some fundamental difference can usually be detected. I should like to think that almost as much as two men have sometimes independently arrived at exactly the same discovery, so Natural Selection, working for the good of each being, and benefiting from all analogous variations, sometimes has two parts modified in almost exactly the same way in two organic beings,
Though in many cases it is very difficult to guess by what transitions the organs came to be in their present condition, yet, considering the very small number of extant and known forms as against the extinct and unknown forms, I have been greatly astounded find how rare an organ occurs, to which some transitional stages easily led. It is certainly not true that new organs often suddenly appear in a class, as if they had been first created for some particular purpose; — and as is already recognized by the old, although somewhat exaggerated, rule of natural history "Natura non facit saltum". We find this accepted in the writings of almost every experienced naturalist; MILNE E DWARDS _put it this way: Nature is extravagant in variations, but stingy in innovations. According to the theory of creation, how should it be? whence should it be that all the parts and organs of so many independent beings, though each created for its own place in nature, are yet concatenated by very gradual transitions? Why wouldn't nature have a leap from one organization to another[p. 222]did? On the other hand, according to the theory of Natural Selection, we can understand it clearly, because it takes advantage of only minute, gradual variations; she can never make a leap, but must advance with the shortest and slowest steps.
Organs of apparently minor importance. ) Since natural selection works for life and death, namely by preserving individuals with advantageous variations and suppressing those with unfavorable deviations of organization, it has sometimes seemed to me very difficult to comprehend the origin of simple parts, the importance of which does not seem sufficient to ensure the preservation always to justify further changing individuals. This difficulty, though of quite a different nature, sometimes seemed to me to be as great as that concerning organs so perfect and compound as the eyes.
In the first place, however, we know far too little of the whole household of an organic being to say what slight modifications may be important to it, and I have in an earlier chapter instances of very slight characters, such as the color of the skin and hair of some Quadrupeds, or as cited the down of the fruits and the color of their flesh, which, insofar as they are relevant to the attacks of insects, or are connected with the sensitivity of beings to external influences, certainly with in Natural Selection come into consideration. The giraffe's tail looks like a man-made fly whisk, and it seems incredible at first that by small successive improvements it should have been gradually prepared for the insignificant purpose of such an instrument. But let us be careful not to express ourselves too firmly in this case, since we know that the existence and mode of distribution of the cattle u. a. animals inSouth America depends upon their ability to withstand the attacks of insects; hence individuals who are reasonably equipped with means of defense against these small ones[p. 223]Enemies are provided would be adept at spreading themselves over new grazing-places with great advantage. Not that large mammals (excepting a few rare cases) are now being devoured by flies; but they are so constantly fatigued and weakened by them, that they succumb to disease, occasional want of food, and the pursuit of beasts of prey in far greater numbers.
Organs of now insignificant importance may at times have been of great value to the first progenitors, and, after earlier slow perfection, may have been inherited in about the same condition by their offspring, though their present usefulness is but insignificant, while harmful deviations from the former structure are caused by natural ones Breeding are constantly prevented. If one observes what an important organ of locomotion the tail is for most aquatic animals, its general presence and use for various purposes in so many terrestrial animals which, by modified swim-bladders or lungs, trace their aquatic descent betray, understand quite well. Once an aquatic animal is endowed with a well-developed rudder tail,
Secondly, we may sometimes attribute great importance to characters which in fact do not belong to them, and which stem from quite secondary causes independent of natural selection. Recall that climate, food, etc. probably have some small influence on the organization; that older characters reappear after the law of return; that interrelationships in development exert an often important influence on the modification of various structures, and finally that[p. 224] Sexual selection has often acted essentially on such outward characters of an animal species as have given the male fighting with others a better weapon, or a special charm in the eyes of the female. Moreover, a change in structure arising from the causes mentioned may at first have often been of no value to the species, but later gained importance in its offspring, placed under new conditions of life and provided with new habits.
I will give some examples to illustrate this last remark. If only green woodpeckers existed and we knew nothing about black and variegated, I would venture to say that the green color is a beautiful adaptation, designed to hide these tree-climbing birds from the eyes of their enemies, that it is therefore a character which is important for the species and has been acquired through natural selection; but as it stands, the coloring is undoubtedly due to quite another cause, and probably to sexual selection. A climbing palm species in the Malay Archipelagoascends to the highest tree-tops by means of excellent tendrils, attached in tufts to the ends of the branches, and this arrangement is undoubtedly of the greatest service to the plant. However, since we see almost similar tendrils on many plants which do not climb, these tendrils may also arise in the case of bamboo from unknown laws of growth, and only later, when the plant underwent other modifications and became a climber, used it to its advantage and have been further developed. The bare skin on the Geyer's head is usually regarded as a direct comfort to the animal, which often burrows in rotten corpses with it; meanwhile we have to be careful with this interpretation, since the scalp of the neatly eating whale cock is also bare.[p. 225]even indispensable; but since sutures of this kind also occur on the skulls of young birds and reptiles, which need only hatch from a broken egg, we may conclude that this mode of formation derives from the laws of growth, and then only occasionally to those in the higher vertebrates way.
We know absolutely nothing about the causes which bring about the slight variations, and feel this most when we reflect on the differences in our domestic animal breeds in other countries, and especially among less civilized peoples who are not concerned with systematic breeding. The domestic animals kept by savage peoples in different regions often have to struggle for their own existence; they may be subject to some degree to natural selection, and individuals of but slightly different constitution sometimes thrive best in different climates. An observer asserts that the ox, with a certain coloring, is more exposed to the attacks of flies, as well as being more susceptible to poisons, so that color thus becomes an object of natural selection. Other observers are convinced that a humid climate promotes hair growth and that horn and hair are equally related. Mountain races are everywhere distinct from lowland races, and mountain regions are likely to act upon the hind legs, and at most the pelvis, inasmuch as these are more engaged there; according to the law of homologous variation, the front limbs and probably the head will also be affected. Also, the shape of the mother's pelvis may exert pressure on the boy's head shape in her womb. It is probable that the more difficult breathing in high mountains also increases the size of the thorax, and this not without affecting still other parts. The effect of lack of exercise on general organization, in conjunction with plenty of food, is probably of still greater importance; and therein lies, as stated by NATHUSIUS recently proved apparently a main cause[p. 226] of the great changes which the various breeds of pigs have undergone. We have, however, far too little experience to consider the comparative importance of the various laws of modification, known and unknown, and I have here mentioned them only to show that, if we are not able, the characteristic differences of our cultivated races to explain which are generally supposed to have arisen by ordinary propagation, we must not overestimate our ignorance of the exact cause of slight analogous differences between species. In this connection I would like to cite the so sharply marked differences between the races of men that some light might perhaps be shed upon the origin of which by assuming a sexual breeding of its own kind; but it would be useless to dwell on it, since I cannot here enter into the details necessary for explanation.
The preceding remarks also cause me to say a few words about the objection recently lodged by several naturalists against the doctrine of utility, according to which all details of culture should be there for the benefit of their possessor. They are of the opinion that very many organic formations exist only for the sake of variety, or to please the eyes of man. If this teaching were correct, it would absolutely have to be fatal to my theory. But I fully admit that some formations are of no immediate use to their possessor. The natural conditions of life are likely to have some slight influence on the organization, whether useful for anything or not. Interrelationships in development undoubtedly also play a very large part, and the useful modification of one organ has often caused useless modifications in other parts as well. In the same way, characters which were previously useful, or which arose through interrelationships in earlier development, or through a completely unknown cause, can subsequently appear[p. 227]reappear in the Laws of Return, though of no immediate use. The effects of sexual selection, in so far as they are based on the fascination of the female, can only be called useful in a more restricted sense. But by far the most important consideration is that the main part of the organization of every being is simply acquired by inheritance, hence, although no doubt every being may be quite well made for its place in the economy of nature, many formations no longer have immediate relations to the way of life of the current species. Thus we can scarcely believe that the webbed foot of the frigatebird or the land goose (Chloephaga Maghellanica) is of any special service to these birds; and we cannot accept that the same bones in the arm of the monkey, in the forefoot of the horse, in the wing of the bat, and in the oar of the seal, bring all these animals a special service. We may confidently regard these formations as inheritance; for doubtless webbed feet were as useful to the progenitor of that goose and frigate-bird as they are to most living water-fowl. Thus we may surmise that the progenitor of the seal did not possess an oar but a five-toed walking or prehensile foot; we may further surmise that the individual bones inherited from one ancestor in the legs of the ape, horse, or bat, have formerly been more useful to their common ancestor or ancestors than they are now to these animals so widely divergent in habits. We may, therefore, suppose these various bones to have been produced by natural selection, formerly as now subject to the laws of heredity, regression, interdependence in evolution, &c. Hence every detail of structure in every living creature (except for a few slight allowances to the influence of natural external conditions) may be supposed to have been of especial service to an ancestor of the species, or to be now, either direct[p. 228]or indirectly, by intricate laws of growth, a special advantage to the descendants of these ancestors.
Natural selection cannot well bring about any variation in one species which has been for the exclusive benefit of another species, though in all nature one species incessantly benefits from the organization of another. But natural selection can often produce, and often in fact produces, such formations as are to the immediate disadvantage of another species, as we see in the fangs of the viper, and in the burrow of the ichneumon, which by the aid of these burrows lays its eggs in the bodies of other living beings introduces insects. If it could be proved that any part of the organization of one species had been formed for the exclusive benefit of another species, my theory would be shattered, because such formation cannot be effected by natural selection. Although many claims are made in natural history writings on this point, I can find none of any weight. Thus the rattlesnake is admitted to possess a fang for its own defense and for killing its prey; but some authors also suggest that she received her rattles for her own detriment, viz., to warn her prey and induce flight. One might as well say, however, that the cat makes the curves of the waves with the end of its tail when it is about to jump in, with the intention of warning the mouse, which has already been sentenced to death. However, I do not have space here to go further into these and other cases. yet I cannot find any among them of any weight. Thus the rattlesnake is admitted to possess a fang for its own defense and for killing its prey; but some authors also suggest that she received her rattles for her own detriment, viz., to warn her prey and induce flight. One might as well say, however, that the cat makes the curves of the waves with the end of its tail when it is about to jump in, with the intention of warning the mouse, which has already been sentenced to death. However, I do not have space here to go further into these and other cases. yet I cannot find any among them of any weight. Thus the rattlesnake is admitted to possess a fang for its own defense and for killing its prey; but some authors also suggest that she received her rattles for her own detriment, viz., to warn her prey and induce flight. One might as well say, however, that the cat makes the curves of the waves with the end of its tail when it is about to jump in, with the intention of warning the mouse, which has already been sentenced to death. However, I do not have space here to go further into these and other cases. that she received her rattles to her own detriment, viz., to warn her prey and cause them to flee. One might as well say, however, that the cat makes the curves of the waves with the end of its tail when it is about to jump in, with the intention of warning the mouse, which has already been sentenced to death. However, I do not have space here to go further into these and other cases. that she received her rattles to her own detriment, viz., to warn her prey and cause them to flee. One might as well say, however, that the cat makes the curves of the waves with the end of its tail when it is about to jump in, with the intention of warning the mouse, which has already been sentenced to death. However, I do not have space here to go further into these and other cases.
Natural selection cannot in any species produce anything injurious to it, acting solely through and for its own advantage. No organ, as P ALEY remarks, can be formed to bring distress and harm to its possessor. A careful consideration of the benefit and harm caused by each part will always show that taken as a whole it is beneficial. Becomes a part in later times with changing living conditions[p. 229]harmful, it is either altered, or perishes the way their myriads perished.
Natural selection strives to make every organic being just as perfect or a little more perfect than the other inhabitants of the same region with whom it has to struggle for existence. And we see that this is the degree of perfection which nature strives for. The natural products peculiar to New Zealand are perfect, one compared with the other; but they are now rapidly retreating before the advancing legions of plants and animals introduced from Europe . Natural selection will not produce absolute perfection; nor, as far as can be judged, do we encounter such a high level in nature. The improvement for the deviation of the light is, as the excellent informant J OH . MÜLLER does not yet fully explain, even in the most perfect of all organs, the human eye. If our reason prompts us to enthusiastic admiration of a multitude of inimitable arrangements in nature, this same reason also teaches us that we can easily err on both sides, other arrangements being less perfect. We can never regard as perfect the sting of the wasp or bee, which, once used against the attacks of various animals, must cause the inevitable death of its possessor, from not getting out of the wound it made because of its barbs , can be withdrawn without dragging the insect's guts after it.
Let us suppose that the sting of the bee existed in a very early ancestral form as a tool for drilling and sawing, as is often the case in other members of the Hymenoptera order, and that its present purpose is related to the sting originally used to produce gallbladder Altered for outgrowths or other poisons, but not at the same time improved, we may perhaps understand why the use of this sting so often causes the insect's own death; because if the fortune to prick[p. 230]useful to the whole bee community, it may meet all the requirements of Natural Selection, though its nature induces the death of the single individuals who employ it. If we marvel at the truly marvelously keen sense of smell by which some males are able to locate their mates, we may then also marvel at the production, for this one purpose, of thousands of drones, which, of the community for each other Purpose utterly useless, destined at last to be killed by their laboring but barren sisters? It may be hard, but we must admire the queen bee's wild, instinctive hatred, which constantly urges her, the young queens, her daughters, to kill immediately after their birth, or perish themselves in the struggle; for unquestionably this is for the good of the community, and maternal love or maternal hate, though fortunately the latter much rarer, is wholly irrelevant to the inexorable principle of Natural Selection. If we compare the various ingenious arrangements by which the flowers of orchids and many other plants are fertilized by insect action, how can we then regard the arrangement in our conifers as equally perfect, by which great and dense clouds of pollen must be produced , so that a few grains of it may be brought to the oak by a favorable breeze? and maternal love or maternal hate, though the latter is happily much rarer, is entirely indifferent to the inexorable principle of Natural Selection. If we compare the various ingenious arrangements by which the flowers of orchids and many other plants are fertilized by insect action, how can we then regard the arrangement in our conifers as equally perfect, by which great and dense clouds of pollen must be produced , so that a few grains of it may be brought to the oak by a favorable breeze? and maternal love or maternal hate, though the latter is happily much rarer, is entirely indifferent to the inexorable principle of Natural Selection. If we compare the various ingenious arrangements by which the flowers of orchids and many other plants are fertilized by insect action, how can we then regard the arrangement in our conifers as equally perfect, by which great and dense clouds of pollen must be produced , so that a few grains of it may be brought to the oak by a favorable breeze?
Summary of the chapter. ) In this chapter we have discussed certain difficulties and objections which oppose my theory. Some of these are of a very serious nature; but I believe that by their discussion some light has been shed on several facts which, on the theory of independent acts of creation, would remain quite obscure. We have seen that species at any given time cannot vary ad infinitum, and are not interrelated through innumerable transitional forms, partly because the process of natural selection is always very[p. 231]is slow, and affects only very few forms at any given time, and partly because the very process of natural selection also usually involves the continual replacement and extinction of previous and intermediate gradations. Closely allied species, now occupying a continuous surface, may often have been formed when the surface was not yet continuous, and the conditions of life did not vary imperceptibly from one place to another. When two varieties have arisen in two parts of a contiguous area, an intermediate variety will often have arisen for an intermediate zone; but, for reasons given, the middle variety will usually have been in smaller numbers than the two varieties conjoined by it,
We have seen in this chapter how cautious one must be in concluding that the most varied habits of life cannot be superimposed, that a bat, e.g. B. could not have originated in the course of natural breeding from an animal which was only able to glide through the air.
We have seen that under changed conditions of life a species may change or diversify its habits, and adopt some customs differing from those of its nearest allies. From this we can understand, if at the same time we remember, that every organic being is urged to live wherever it can live as it came to be, that there are web-footed land-geese, terrestrial woodpeckers, diving thrushes, and petrels with the manners of the Give auk.
Though the opinion that an organ so perfect as the eye can be produced by natural selection is more than sufficient to make anyone falter, yet there is no logical impossibility that any[p. 232]organ under changing conditions of life, through a long series of gradations in its composition, each of which is useful to the possessor, finally attaining every conceivable degree of perfection by the way of Natural Selection. In cases where we know of no intermediate states, we must beware of concluding that such never existed; for the homologies of many organs and their intermediate stages show that wonderful changes in their functions are at least possible. So e.g. For example, a swim bladder has apparently been transformed into an air-breathing lung. In particular, transitions must often have been greatly facilitated,
We are far too ignorant of most cases to assert that a part or organ is immaterial to the growth of a species, and that variations in its formation could not have been effected by natural selection by slow accumulation. But we may confidently suppose that many variations are wholly occasioned by the laws of growth, and, at first without any benefit to the species, have afterwards been used to the advantage of more modified offspring of that species. We may further believe that a part highly important to earlier forms has been retained by later forms (like the tail of an aquatic animal by the terrestrial animals descended from it), though appearing so unimportant to them,
Natural breeding in no species produces anything that[p. 233]would be for the exclusive benefit or harm of another; though it may produce parts, organs, and excretions, which, though very useful to others, and even indispensable, or highly perishable, are in all cases at the same time useful to the possessor. In any well-populated region, natural selection must necessarily work towards improvement or strengthening for the struggle for existence, chiefly as a result of the competition among the inhabitants, but only according to the standard applicable to that region. Hence the inhabitants of one region, and usually the smaller one, often have to give way to those of another and generally larger one. For in the larger area more individuals and more differentiated forms will have existed, the competition will have been stronger and consequently the goal of perfection would have been set higher. Natural selection will not necessarily produce absolute perfection, and, as far as our limited faculties can judge, such is nowhere to be found.
According to the theory of natural selection, the full meaning of the old belief in natural history " Natura non facit saltum " can be understood. This statement is not quite correct if we only consider the present inhabitants of the earth, but according to my theory it must be perfectly true if we include all beings of past times.
It is generally accepted that all organic beings have been formed according to two great laws: unity of type and adaptation to the conditions of existence. By unity of type one understands the agreement in the basic plan of structure as we find it in the beings of a sub-kingdom, and which is quite independent of their way of life. According to my theory, unity of type is explained by unity of descent. Adaptation to the conditions of life, so often employed by the celebrated Cuvier, is fully implicit in my Principles of Natural Selection. For natural selection works only in so far as it keeps the variable parts of every being organic and[p. 234]inorganic living conditions either adapt now or have adapted in time periods long past. These adaptations may in some cases be aided by use and disuse, modified by direct action of the external conditions of life, and are in all cases subject to the various laws of development. Hence the law of adaptation to the conditions of life is in fact the higher one, inasmuch as, by virtue of the heredity of earlier adaptations, it includes that of the unity of the type.
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