NATURAL SELECTION.
In his introduction, Mr. Darwin tells us that "when on board H. M. S. Beagle as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to throw some light on the origin of species." In the chapter on Geographical Distribution, he says that "neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions." Thus, the plants and animals of South America, between latitudes 25° and 55°, are very different from those of Australia and South Africa; and yet the physical conditions of these three countries, within these limits, are very similar, while, notwithstanding the great differences of the physical conditions north of 25° and south of 35°, the plants and animals of these parts of South America are very similar. The existence of lofty mountain-chains, great deserts, etc. acts as a barrier to the free dispersion of plants and animals, and, therefore, is of great importance in reference to Geographical Distribution; thus, the life of opposite mountain-chains is often quite different. This is equally true of the ocean life on opposite sides of a continent: thus, the marine animals of the north side of the Isthmus of Panama are very different from those of the south side, whereas similar fishes are found in as remote waters as the Pacific and Indian oceans,—there being no obstacle to their free dispersion. The relation of the living animals to those found fossil in the same countries is very significant in this respect, the Apteryx of New Zealand representing the gigantic Dinornis, the Armadillo and Sloth the extinct Glyptodon and Megatherium. Further, in reference to Geographical Distribution, the fact of the plants and animals of islands being like those of the nearest island or mainland is as important to the Geologist as to the Botanist and Zoologist. Thus, Mr. Wallace explains the similarity of the plants and animals of Sumatra, Java, Borneo, etc. to those of southern Asia by supposing that these islands once formed part of that continent, being connected with it by Malacca; while the Celebes, Moluccas, New Guinea, resembling in their flora and fauna Australia, are regarded as forming with it another continent, the islands of Bali and Lombok indicating the limits of these ancient continents. Mr. Wallace says, in crossing over the straits separating these islands, "we may pass in two hours from one great division of the earth to another, differing as essentially in their animal life as Europe does from America." The study of South America and the Malay Archipelago suggested to Messrs. Darwin and Wallace their explanation of the Geographical Distribution of plants and animals through what Mr. Darwin calls Natural Selection, which may be expressed as follows:
Plants and animals struggle for existence. The immediate descendants are never absolutely like their parents; while remote posterity often differs considerably from them.
Those plants and animals whose modified organization gives them an advantage over those not so favored survive, or are naturally selected, and transmit their modifications to posterity.
Let us examine these statements, and try to explain how the conclusion follows.
STRUGGLE FOR EXISTENCE.
Every one knows that the life of an individual plant or animal depends on a proper supply of food, is affected by changes of climate, and is constantly endangered by disease and enemies; few are, however, aware of the extent to which individual life is dependent on the existence of some other kind of life, and of the extremely complex nature of the struggle for existence. Thus, according to Prof. Haeckel, " There are small oceanic islands whose inhabitants live essentially on a species of Palm. The fructification of these Palms is effected principally through Insects, who carry the pollen from the male to the female Palm. The existence of these useful Insects is endangered through Insect-feeding birds, who in turn are pursued by Rapacious birds. But the Rapacious birds often succumb under the attacks of a small parasitic Mite, which develops by millions in their feathery coats. These small, dangerous Parasites can be killed through parasitic Fungi. Fungi, Rapacious birds, and Insects in this case would favor, Bird-mites and Insect-feeding birds, on the contrary, would endanger, the growth of the Palms, and consequently of men." Thus the existence of entire populations may be indirectly dependent on the presence of a highly insignificant plant or animal form. If one considers the millions of eggs laid by fishes, and that a pair of elephants, the slowest of breeders, would reproduce in five hundred years fifteen millions, the importance of the struggle for existence in checking over-population will be appreciated. According to Mr. Darwin, the red clover never produces seed if the humble-bees be prevented from visiting it. For the bee, in sucking the honey out of the flower, brings the pollen in contact with the pistil, and by this means the clover is fertilized. Now, it is well known that the bees are destroyed by the field-mice, and that the number of mice depends on the number of cats; hence, if the cats were destroyed the field-mice would increase and destroy the bees, in which case the clover would produce no seed, and the cattle would soon be deprived of a most important article of food. The same author calls attention to the fact of cattle determining the existence of trees: "Here there are extensive heaths, with a few clumps of old Scotch firs, on the distant hill-tops: within the last ten years large spaces have been inclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. When I ascertained that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view whence I could examine hundreds of acres of the uninclosed heath, and, literally, I could not see a single Scotch fir, except the old planted clumps. But on looking closely between the stems of the . heath, I found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees; and one of them, with twenty-six rings of growth, had, during many years, tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was inclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren, and so extensive, that no one would ever have imagined that cattle would have so closely and effectually searched it for food. Here we see that cattle absolutely determine the existence of Scotch fir. But in several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they" swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly which lays its eggs in the navels of these animals when first born," the new-born dying in consequence of these inroads. As illustrating the struggle for existence, Prof. Haeckel calls attention to the effect of placing goats or pigs on one of the isolated oceanic islands uninhabited by man. These animals ran wild, and meeting no enemies, and finding at first plenty of food, increased in such surprising numbers that they killed the other animals and plants. Thus, in the course of time, the whole island was nearly depopulated, the goats or pigs even dying out for want of food. In some cases dogs were let loose, after the island hud been overrun by goats or pigs. The dogs, finding an abundance of food at first, increased so rapidly that their very numbers made food so scarce that they finally died out.
Many other interesting examples of the struggle for existence might be mentioned; those just noticed suffice, however, to call attention to the important relation existing, in this respect, between plants and animals. We turn now to a consideration of the facts of Inheritance and Variation.
INHERITANCE.
Not only does the offspring resemble the parent in the manner of its growth, in the form of its body and general appearance, but, as is well known, mental peculiarities are also inherited, the development of particular talents for music, painting, etc. being conspicuous in certain families: thus, the Bach family have numbered twenty distinguished musicians, the family of Weber upwards of forty. Less pleasant peculiarities, as those of diseases of all kinds, are well known to reappear in posterity both at the same time and in the same place as they first appeared in the parent. In a word, generally speaking, "Like begets like." This expression, however, is not absolutely correct, since the members of every family differ more or less in the color of their eyes and hair, in their complexion, dispositions, etc. The same species of trees differ as regards the size of their stem, number of their branches, leaves, flavor of their fruit, etc. Let us examine now, a little in detail, some of these variations, and attempt to indicate their probable causes.
VARIATIONS.
The quantity and quality of the food are known to modify animals and plants. Thus, what a marked difference is produced in the habit of our domestic animals, who are fed daily, and of wild ones, whose means of subsistence are so precarious! The quality of the food modifies, as well as the quantity, the tissues: thus, richly azotized food develops little fat, poorly azotized food, on the contrary, a great deal. The farmer requiring fine wool supplies his sheep with different food from that which he gives wishing to obtain good meat. Notice the effects of a rice diet as seen on the Chinese, and of a beef one on the English. Climate is an important element in the production of variations: thus, plants growing in dry, warm, and sunny places offer a very different aspect from those of moist, cool, and shady spots. Plants that at the sea-side exhibit thick, fleshy leaves, in hot, dry places develop haired ones. The crowding of trees has the effect of making the stem tall, while it diminishes the foliage; whereas the foliage of the isolated tree expands, the stem being comparatively short. We see, therefore, that the social state, so to speak, is of importance in modifying forms. The use and disuse of organs produce most marked effects: thus, the wings of the domestic duck are lighter than those of the wild one, whereas the legs of the domestic duck are heavier; the difference being caused undoubtedly by the different habits of these animals. The rudimentary condition of the muscles moving the external ear, in domestic animals, is no doubt due to their disuse; domestic animals not being, like wild ones, continually on the watch for prey or enemies. Gymnasts illustrate well the effect of using the muscles, sedentary persons of their disuse. The development of the mental faculties by their use is well seen in the domesticated Dog and Horse as compared with these animals when in a wild condition; while their degeneration through want of use has been noticed in the domesticated Rabbit, whose senses are not so keen as those of the wild one. Parasites are interesting in this respect: thus, the young of many parasites lead a free, active life, exhibiting often a complex organization. In the course of time, however, in adopting a parasitic mode of life they lose many organs, or retain them only in a rudimentary condition. The greater development of the bones, muscles, and nerves of the right hand as compared with the left, is due, no doubt, originally to greater use. This variation, like many others, has been inherited by posterity, since the new-born child offers the same difference in its hands. The thickened skin on the soles of the feet and the palms of the hand (seen also in the new-born child) has no doubt been acquired in the same way. The eye which is most used in microscopy becomes near-sighted, the other eye far-sighted. It must be borne in mind, also, that variations beget variations; the different organs of a plant or animal being so correlated that it is impossible to modify one organ without sooner or later some other organ becoming affected: thus, the increased flow of blood to a part, through continual and violent muscular action, may finally produce hypertrophy of the heart. There are many variations, however, arising through the correlation of organs, which cannot be so readily explained: thus, certain Pigs and Dogs, who lose their hair when taken to warmer climates, have their teeth affected. The Edentata are so called from the peculiar character of their teeth; but the skin-covering of many of them, like the Armadillo and Pangolin, is equally remarkable. Short and compressed heads accompany short limbs, as seen in Pigs and Cattle. The horned animals are without incisor and canine teeth, as seen in the Ruminants, etc.; while those that have these teeth (like Pigs and the Musk-deer) never exhibit horns. With the long legs of Wading-birds (Heron and Stork) are associated long beaks. Dark-skinned, darkhaired, and brown-eyed Europeans are less susceptible to tropical diseases, and therefore more easily acclimatized, than those with light skin, blond hair, and blue eyes. These examples illustrate the important principle of one variation entailing another through the correlation of organs. The different modifications that we have mentioned are variations appearing in the parent, and often transmitted to the offspring. But there are also variations which first appear in the posterity, such as monstrosities, the difference of the sexes, etc. Of these variations it is often difficult to say whether they are produced by causes acting directly on the parents, or directly on posterity, or indirectly through the parents on posterity. While the causes of many variations as well as their transmission to posterity are obscure or unknown, it seems very probable that changes in Nutrition are the causes of all variations, the term Nutrition including the effects of Food, Climate, Social Relations, Use and Disuse, Correlation of Organs, etc.; while the facts of Inheritance are to be explained by the laws of Generation, of which as yet few are known. No doubt at some future day Nutrition and Generation will be shown to be simply physical and chemical phenomena. However that may be, the important fact is that "all organic individuals become in the course of their life, through adaptation to different conditions of existence, unlike one another, although the individuals of one and the same species remain mostly very similar."
SURVIVAL OF THE FITTEST.
Every one is aware that favorite breeds of cattle, many beautiful flowers, particular kinds of horses, such as the dray-horse, race-horse; etc., are not found in a wild state, but that these forms have been gradually produced. Let us examine the means by which this end has been accomplished. Suppose, for example, a gardener wishes so to modify some particular white flower that in time it will exhibit a striking scarlet color. He looks carefully among the particular flowers until he finds one which offers a trace of red; he plants the seeds of this flower, and from their posterity he selects the reddest flowers. Continuing to select the reddest flowers, and planting their seeds alone, finally the gardener succeeds in obtaining one of a scarlet color. The success of the gardener's operation depends upon the fact of there appearing, among the flowers which are usually white, one exhibiting a faint red color, and upon the fact of posterity inheriting from their parents a variation which they transmit in turn to their offspring, this variation becoming more marked as it is transmitted from generation to generation. An equally good illustration of this principle is the often-quoted instance of Seth Wright, the Massachusetts farmer, who, noticing that one of his rams, with a long body and short bandy legs, could not jump over the fences, concluded that it would be a good thing to breed with this ram alone, and to his great satisfaction soon obtained a race of sheep characterized by the peculiarity of a long body and short bandy legs. Indeed, although the subject of inheritance is still theoretically obscure, practically it is so well understood that Sir John Sebright "can produce in three years a given feather, but that he requires six years in order to obtain a particular kind of head and beak." Now we know that variations appear among wild animals, and that these variations are transmitted to their posterity. Is there then also a selection in nature which brings about the same results as that produced by man's selection? Suppose, for example, a number of plants are growing in a dry place, it is evident that those plants whose leaves are most thickly haired will be favored in the struggle for water, since the hairs are useful in taking up moisture. These plants will therefore survive and reproduce their kind, while those whose leaves are deficient in hairs will die out. But in the next generation some of the plants will be characterized by still thicker hairs; these will therefore be preserved and procreate; but in the course of generations plants are produced through this Natural Selection which differ very considerably from the parent stock, not only in the hairing of the leaves, but in other peculiarities, as one variation sooner or later entails another. Thus the moisture taken up by the hairs furnishes a large amount of nutriment, but if the nutriment is increased the flowering organs diminish; but this effect in the struggle for existence will bring about other variations, and so on indefinitely." The wingless condition of so many Madeira beetles is mainly due to the action of natural selection, but combined probably with disuse. For during many successive generations, each individual beetle which flew least, either from its wings having been ever so little less perfectly developed, or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight would oftenest have been blown to sea, and thus have been destroyed." Through the Survival of the Fittest, by Natural Selection, we see why animals resemble in color, etc. their surroundings or the places they live in. Thus, the Plant-lice and many insects are green, like the leaves they live upon. The Jumping Mouse, Fox, Lion, and Gazelle are yellow or yellowish-brown, like the sands of the desert they frequent. The Polar Bear, living on ice and snow, is white or gray; but as the summer advances and the snow passes away, leaving the dark ground exposed to view, the Bear changes his skin to a brown or black, assuming again, as winter returns, its whitish hue. Mr. Darwin explains these striking facts by showing that the harmonizing of the color of an animal with its surroundings is useful to it. For those animals being unobserved are favored in the struggle for existence, seizing more easily their prey, or escaping from their enemies more readily, than those not so favored. Mr. Wallace, speaking of the butterfly Kallima parapleta, says, "At length I was fortunate enough to see the exact spot where the butterfly settled, and, though I lost sight of it for some time, I at length discovered that it was close before my eyes, but that in its position it so closely resembled a dead leaf attached to a twig as almost certainly to deceive the eye even when gazing full upon it." In reference to this subject, Prof. Haeckel notices the Helmichthys, fishes whose bodies are so transparent that one can read a book through them. The Carinaria among the Mollusca, the Salpa among the Worms, many of the Jelly-fishes, are either bluish or colorless as the water they live in. The transparent glass-like color of these animals who live on the surface of the open sea is evidently of service to them in catching the objects of their prey or avoiding their enemies. Suppose now the remote ancestor of one of these animals to have been slightly transparent, a little more so than the individuals of the same species, it would have been favored in the struggle for existence, and would have survived. Transmitting this useful peculiarity, its posterity would be still more transparent. Finally, in the course of generations, almost perfectly transparent animals would be produced. Prof. Cope observes, "The gray sand hue so well adapted for concealment is universal, with few variations, in the reptiles of the Tartar and Arabian deserts, the Great Sahara, and the sands of Arizona and California. There is also a tendency to produce spiny forms in Such places; witness the Stellios and Uromastix and Cerastes of the Sahara, the Phrysonomas and Horned Rattlesnake of Southwestern America. The vegetation of every order, we are also informed, is in these situations extremely liable to produce spines and thorns."
Every one is aware of the great difference in size and color exhibited by the male and female of birds, butterflies, etc., of male animals being armed with weapons, like the horns of deer, the cock's comb, etc. Mr. Darwin supposes these organs to have arisen through what he calls Sexual Selection. Thus, at breeding-time the number of male deer exceeds that of the female; hence there is invariably a fight, and the deer with the biggest horns gets the better of his rivals: naturally their posterity will be characterized by large horns. This process, continued through generations, finally results in the production of the antlers of the male deer. But, as Mr. Herbert Spencer observes, large horns require large muscles to move the head, large muscles must be supplied with sufficient nutriment, which is brought to them by large arteries, which necessitates a powerful heart, and so on indefinitely. The voice of the singing birds is supposed to have arisen in the same way, for of the male birds those who sing best are chosen by the females for their mates. The voice is therefore continually improved from generation to generation. The male Crickets, Grasshoppers, Katydids are equally remarkable for the noise they can make. The incessant "Katydid she didn't" is produced by one wing being played on by the other wing, like a fiddle and bow. "All observers agree that the sounds serve either to call or excite the mute females;" and Mr. Darwin quotes Mr. Bates as stating that the male of the European field-cricket "has been observed to place itself in the evening at the entrance of its burrow, and stridulate until a female approaches, when the louder notes are succeeded by a more subdued tone, whilst the successful musician caresses with his antenna; the mate he has won." Ornaments of the male animal, like the cock's comb, the peacock's tail, the gorgeous plumage of the paradise bird, the brilliant color of the male butterfly, are made use of, like the weapons and musical tones just mentioned, in obtaining the female. The old Spartan principle of killing the deformed and sickly, which resulted in the production of a magnificent race of men, is the action of Sexual Selection applied to man. Necessarily the offspring will exhibit marked improvement in beauty of form, development of talent, and powers of defense, if the fathers are always selected from those who approach nearest the standard of excellence.
Having illustrated now, we hope sufficiently well, the selection brought about by man and nature, let us see how they differ and in what they agree. Man selects knowingly, with an object; making use of variations, he modifies for his own advantage. Nature eliminates blindly, without an object, the organisms surviving being better fitted for existence through some advantage. Thus the Massachusetts farmer knowingly made use of the variation of a long body and short bandy legs, exhibited by one of his rams, to produce a particular race of sheep. But suppose the conditions of existence had been such that the short-legged sheep had some advantage over the long-legged ones in the struggle for existence, the favored ones would have survived, and nature blindly would have done in the long run what the farmer did in a few generations. A similar case would be that of a farmer who, having black and white pigs, wanted black pigs only. To attain this object he would knowingly separate the pigs, and breed from the black pigs alone. But if the farmer lived in Florida, and would turn his pigs in the woods, nature would blindly bring about the same result, since the white pigs would soon die, it being well known that pigs eat "the paint roots (Lachnanthes ), which color their bones pink, and which cause the hoofs of all but the black varieties to drop off:" hence the squatters say "we select the black members of a litter for raising, as they alone have a good chance of living." Suppose man to be heartless enough to abolish all hospitals, almshouses, etc., soon nature would eliminate, as Sparta got rid of, the sickly and deformed, the result being the survival of the fittest. We have seen that there is a most complex struggle for existence, that while like begets like, plants and animals vary in their organization; it follows, necessarily, that those organisms whose variations give them an advantage in the struggle for existence will survive, or be naturally selected, while those not so favored will die out. We see, therefore, that Natural Selection neither implies the existence of a Natural Selector nor is the Survival of the Fittest effected by chance. The facts of Inheritance are to be explained by the laws of Generation, those of Variation by Nutrition. The Struggle for Existence is caused by the number of individuals that are born being out of all proportion to the size of the earth they live in. The ever-changing conditions of Nature have the effect of eliminating the conservative kinds of life, while the plastic organisms survive and transmit their peculiarities to posterity. These variations become more marked from generation to generation, until finally, in the course of ages, there result very different forms of plant and animal life.
A good illustration of this whole subject is the history of the Siredon and Amblystoma. The Siredon lichenoides (Fig. 65) is a perennial gill-breathing Batrachian reproducing Siredons; the Amblystoma mavortium (Fig. 66) breathes by lungs and reproduces Amblystomas. These forms were naturally supposed to be distinct kinds of Batrachians; but the experiments of Prof. Marsh and Prof. Dumeril have demonstrated that changing the conditions of existence has the effect of metamorphosing the Siredon into an Amblystoma. The importance of these experiments may be appreciated by supposing that Tadpoles reproduced Tadpoles in Nebraska or Mexico, and Frogs reproduced Frogs in New Haven or Paris, and that anatomists regarded the Tadpole as an entirely distinct Batrachian from the Frog; but removing the Tadpoles to New Haven or Paris, and changing the conditions of existence, the Tadpole turned into a Frog: our hypothetical case is exactly that of the Siredon and Amblystoma. As Professors Marsh and Dumeril developed a lung-breather, the Amblystoma, from a gill-breather, the Siredon, so we believe nature to have developed the lung-breathing Frogs from gill-breathing tadpole-like animals. Thus, in remote time, the conditions of existence changing, some of these tadpole-like animals changed into Frogs, while others remained unmodified and reproduced tadpole-like animals. But as the development of the individual Frog is the epitomized history of the race to which it belongs, the developing Tadpole, or the transitional stages of the Frog, are permanently represented by animals like the Salamander and Proteus.
An important consequence of the Struggle for Existence is the Division of Labor so characteristic of man, the higher animals, and plants. Savages supporting themselves by hunting and fishing, while often acting in concert, are, however, not dependent upon one another, so that the sudden death of even many individuals does not cause any inconvenience to the rest of the tribe. But in the civilized state, where the crowding together of people diminishes the means of subsistence and increases the number of rivals, the Struggle for Existence soon differentiates the population into growers of corn, hewers of wood, and carriers of water, as every one cannot work at the same trade, and the dependence of one upon another becomes very great. This is immediately seen if we consider the confusion that would arise in a city if the butchers and bakers were suddenly to die. But the community in general is not only differentiated by the Struggle for Existence into divers interests, but, sooner or later, the individuals are affected in the same way. For the individual whose organization is most specialized and whose functions are many is better fitted to maintain himself against the changing conditions of life than one whose organization is more simple. But we have seen that one variation entails another, and that the peculiarities of the parents are transmitted to their offspring: hence in the course of generations the organization becomes extremely complex. Thus the Division of Labor is carried out to such an extent in the organization of the human body that it requires volumes to describe the anatomy of man. The Division of Labor, or a complex organization, does not necessarily follow from the Struggle for Existence; for there are animals who, when young, lead a free active life, and have quite a complex organization, but, growing older, they adopt a parasitic mode of life, and then lose many of their organs through disuse. Prof. Haeckel aptly observes, "The traveler lightens his journey who throws away his pack." So of many parasites: the one who first gets rid of any useless muscles or nerves will have the best chance of surviving; complex conditions of existence bring about, sooner or later, complex organization, while simple structures are the result of simple conditions of existence. Supposing this view of Nature to be correct, the plants and animals that first appeared on the earth ought to have been simply and lowly organized, the later ones highly complex. In our chapter on Geology we have shown that such is the case,—that there has been a progress from the lower to the higher forms of life, accompanied at the same time by retrograding metamorphoses, as seen in parasites, etc.
While Natural Selection is generally admitted to be a sufficient cause for the production of unimportant variations, it is often objected that important structures, such as the skeleton, could never be modified by such a process. Mr. Darwin, however, has shown that the skeleton is as susceptible to modification as any other part of the organization. Thus, the different kinds of pigeons, supposed unanimously by " fanciers" to have descended from different ancestors, but which are now known to be the posterity of the Rock Pigeon, offer great variations in their skeleton, as in the number of their vertebrae and ribs, in the character of the breast-bone, merry-thought, lower jaw, and bones of the face. All zoologists admit that the various kinds of rabbits have descended from a common stock; and yet the greatest difference is seen in the size, shape, and form of the skull, in the character of the backbone, etc. But not only have the changing of conditions and the domestication of animals modified the skeleton, which is regarded by anatomists as one of the most constant of characters, but the viscera and all other parts of the organization have been affected. We do not regard, therefore, the objection of Natural Selection not being a sufficient cause of change as of any weight. The fact of Hybrids often not breeding is regarded by many as an important objection. The case of the mule not breeding is usually referred to. This objection does not seem to us to amount to much, as it is well known that the Porto Santo rabbit, which is the offspring of the European rabbits placed on that island in 1419, will not breed now with the posterity of its European ancestor. Further, it does not follow, because mules are unreproductive, that all other hybrids have been, and will be. Thus, the Lepus Darwinii, originally resulting from the crossing of the Rabbit and Hare, now reproduces its kind, the animal being half Hare half Rabbit. According to Prof. Haeckel, the pairing of the male Goat and female Sheep is very common in Chili, their progeny being fertile; while the Ram and female Goat rarely pair, and then without offspring. It is well known that some animals when confined in menageries, etc. will not breed. We see, therefore, upon what slight differences reproduction depends.
The absence of links between the different forms of plants and animals is often urged as an objection to the theory of the Evolution of Life. The not finding of links is due very often to not looking for them in the right place. Thus, a pigeon-fancier, not finding a link between the Carrier and Pouter, might have argued some years ago that they had descended from the primitive Carrier and Pouter, of whose origin he knew nothing. But it is well known now that these pigeons are the posterity of a common ancestor, the Rock Pigeon. Hence the transitional forms are between the Rock Pigeon and the Carrier, between the Rock and the Pouter. We have tried to show that the Struggle for Existence produces a divergence of character, so that in the course of time the posterity differs greatly from the parent stock. Now, if the intermediate animals die out, forms are left which have little in common with existing animals. In this manner may be explained the existence of such isolated unique forms as the Elephant, Sloth, Giraffe, so readily distinguished by their striking peculiarities. The Capuchins, among the South American monkeys, on the contrary, exhibit such a number of varieties, species, and genera that it is almost impossible to classify them, the transitional forms being so numerous. Many have argued that too much time is required in the development of the animal and vegetal kingdoms through the Survival of the Fittest. Physical and Geological science cannot at present assign any definite age to the Earth; and, from the rate at which deposits are formed at the present day, millions and millions of years would have passed away in the formation of the Aqueous Rocks. So that, while admitting the looseness of the data, we feel that we are less likely to err in assuming an amount of time practically unlimited for the development of life than if we attempt to fix a definite limit. It is often asked, How could the instincts of animals and man arise through a process like Natural Selection? The manner in which the young uneducated Pointer acquired the instinct of pointing explains the origin of all instincts. The original Pointer was taught to point, and in the course of generations, this peculiarity being inherited, the pointing became instinctive. All of our ideas have arisen in the same way, mind being the impressions of the brain derived from the external world through the medium of the senses. If there really be what metaphysicians call "a priori ideas," originally they have been derived a posteriori; that is, these ideas were originally derived by the parent organism, and later inherited by posterity. Finally, to many persons, complex structures like the eye and ear are insuperable objections to the theory of Natural Selection, it seeming incredible to those who are unacquainted with Comparative Anatomy that such organs could have arisen through the Survival of the Fittest caused by the action of a blind, objectless, working Nature. The eye is usually studied in a most developed state, as in man, for example; but the visual organ of some of the lower animals is only a pigment spot, more or less sensitive to the rays of light, but incapable of forming the image of an external object. As we ascend in the scale of life, we notice there is added to this pigment spot a sensitive nerve, and as we gradually progress there appears the lens, a light-refracting organ, which, collecting the rays of light in a focus, delineates the image of an external object. Still more highly organized animals exhibit additional media of service in transmitting the light, a complex retina for receiving the image, and special arrangements for the accommodation of the eye to distances. By glancing, therefore, at different animals, we see the eye in different stages of perfection. But the gradual development of the eye of man offers a series of transitional stages, which are permanently represented by the eyes of the lower animals. Now, if the eye has gradually been perfected through the Survival of the Fittest, we understand why the transitional stages in the development of the eye of man have permanent representatives in the eyes of the lower animals; but if the eye has been created for the purpose of seeing, we neither understand why such an extraordinary method is adopted in its formation nor why it is not a perfect optical instrument. The same reasoning will apply to the human ear as well as to the eye, they both beginning as depressions in the skin, which later, closing, form the primitive eye and ear vesicles.
RESUME.
We tried to show in the chapters on Zoology, Botany, Geology, and Embryology, that the structure of plants and animals, their petrified remains, and their manner of development, are explained by supposing that life has evolved, that there has been a gradual development of the higher form? of life from the lower, accompanied here and there by a retrograding metamorphosis. In the early part of this chapter we called attention to the facts of Geographical Distribution not being consistent with a theory that supposed plants and animals had been created for special localities, but that they could be explained by supposing that life migrated from place to place, being more or less modified from time to time by the new conditions of existence, natural barriers being often the cause of the great difference exhibited by plants and animals living under similar conditions. The conclusion of an Evolution of Life, arrived at by a comparison of many biological generalizations, we then tried to show was the necessary consequence following from the Struggle for Existence combined with the effects of Inheritance and Variation: the resultant of these three forces being what Mr. Darwin calls Natural Selection. It must be remembered that Natural Selection does not explain the facts of Inheritance and Variation, but follows from them and the Struggle for Existence. The facts of Inheritance and Variation seem to depend upon Generation and Nutrition, which are chemical and physical phenomena still involved in much obscurity. Before leaving the subject, it seems proper to mention, as it does not appear to be generally understood, that the theory of the Evolution of Life may be accepted as true, and yet Natural Selection not be considered as a sufficient explanation. Suppose, now that the attention of naturalists has been drawn to the theory of Evolution, that most careful observations are made in reference to this subject, and that all biologists become convinced in time that plants and animals gradually change, the flora and fauna of a remote future differing very considerably from those of the present day,—the theory of the Evolution of Life might be demonstrated, and yet it might be shown that Natural Selection did not entirely produce it, or indeed the cause might still remain unknown. Let us repeat, then, that whatever may be thought of the causes advanced, as sufficient to bring about a development of life, the theory of Evolution remains the only explanation of the most important generalization of the comparative anatomy of plants and animals, their Paleontology, Embryology, and Geographical Distribution.