Part 7

That portion of the theory of development which maintains the common descent of all species of animals and plants from the simplest common origin, I have already stated with full justice should be called Lamarckism. Progress is recognized by all scientists to be a law of nature. Some of the more important facts which sustain the theory of development, I propose now to present as briefly as possible.

RUDIMENTARY ORGANS.

One of the strongest arguments in favor of the hypothesis of a genetic connection among all animals (including man), at least among all those belonging to the same great types, is the presence of rudimentary parts. By rudiments in anatomy are meant organs or structures imperfectly developed, so as to be almost or entirely without functional use. "Each of them represents in germ, as it were, in one animal (or plant), that which is perfect and useful in another type."

For a few examples: The little fold of caruncle at the inner margin of the eye in man, represents the nictitating membrane of birds. Eyes which do not see form a striking example. These are found in very many animals which live in the dark, as in caves or underground. Their eyes are often perfectly developed but are covered by a membrane, so that no ray of light can enter and they can never see. Such eyes, without the function of sight, are found in several species of moles and mice which live underground, in serpents and lizards, in amphibious animals (proteus, cæcilia) and in fishes; also in numerous invertebrate animals which pass their lives in the dark, as do many beetles, crabs, snails, worms, etc.

Other rudimentary organs are the wings of animals which cannot fly. For example, the wings of the running birds, like the ostrich, emeu, cassowary, etc., the legs of which become exceedingly developed. The muscles which move the ears of animals are still present in man, but of course are of no use; by continual practice persons have been able to move their ears by these muscles. The rudiment of the tail of animals which man possesses in his 3-5 tail vertebræ, is another rudimentary part--in the human embryo it stands out prominently during the first two months of its development; it afterwards becomes hidden. "The rudimentary little tail of man is irrefutable proof that he is descended from tailed ancestors." In woman the tail is generally, by one vertebra, longer than in man. There still exists rudimentary muscles in the human tail which formerly moved it.

Another case of human rudimentary organs, only belonging to the male, and which obtains in like manner in all mammals, is furnished by the mammary glands on the breast, which, as a rule, are active only in the female sex. However, cases of different mammals are known, especially of men, sheep and goats, in which the mammary glands were fully developed in the male sex, and yield milk as food for their offspring. The vermiform appendix of the large intestine in man, is another illustration of a part which has no use, but in one marsupial is three times the length of its body. The rudimentary covering of hair over certain portions of the body, is not without interest. Over the body we find but a scanty covering, which is thick only on the head, in the armpits, and on some other parts of the body. The short hairs on the greater part of the body are entirely useless, and are the last scanty remains of the hairy covering of our ape ancestors. Both on the upper and lower arm the hairs are directed toward the elbow, where they meet at an obtuse angle--this striking arrangement is only found in man and the anthropoid apes, the gorilla, chimpanzee, orang, and several species of gibbons. The fine short hairs on the body become developed into "thickset, long, and rather coarse dark hairs," when abnormally nourished near old-standing inflamed surfaces. The fine wool-like hair or so-called lanugo with which the human foetus, during the fifth and sixth months, is thickly covered, offers another proof that man is descended from an animal which was born hairy, and remained so during life. This covering is first developed during the fifth month, on the eyebrows and face, and especially around the mouth, where it is much longer than that on the head. Three or four cases have been recorded of persons born with their whole bodies and faces thickly covered with fine long hairs. Prof. Alex. Brandt compared the hair from the face of a man thus characterized, aged thirty-five, with the lanugo of a foetus, and finds it quite similar in texture. Eschricht has devoted great attention to this rudimentary covering, and has thrown much light on the subject. He showed that the female as well as the male foetus possessed this hairy covering, showing that both are descended from progenitors, both sexes of whom were hairy. Eschricht also showed, as stated above, that the hair on the face of the fifth month foetus is longer on the face than on the head, which indicates that our semi-human progenitors were not furnished with long tresses, which must therefore have been a late acquisition. The question naturally arises, is there any explanation for the loss of hair covering?

Darwin is of the opinion that the absence of hair on the body is, to a certain extent, a secondary sexual character; for, in all parts of the world, women are less hairy than men. He says: "Therefore we may reasonably suspect that this character has been gained through sexual selection." As the body in woman is less hairy than in man, and as this character is common to all races, we may conclude that it was our female semi-human ancestors who were first divested of hair.

Professor Grant Allen has given much study to the subject of the loss of hair in the human being; and his investigations are worthy of careful consideration. He shows conclusively that those parts of an animal which are in constant contact with other objects are specially liable to lose their hair. This is noticeable on the under surface of the body of all animals which habitually lie on the stomach. The soles of the feet of all mammals where they touch the ground are quite hairless; the palms of the hands in the quadrumana present the same appearance. The knees of those species which frequently kneel, such as camels and other ruminants, are apt to become bare and hard-skinned. The friction of the water has been the means of removing the hair from many aquatic mammals--the whales, porpoises, dugongs, and manatees are examples.

As the back of man forms the specially hairless region of his body, we must conclude that it is in all probability the first part which became entirely denuded of hair. The gorilla, according to Professor Gervais, is the only mammal which agrees with man in having the hair thinner on the back, where it is partly rubbed off, than on the lower surface. Du Chaillu states that he has "himself come upon fresh traces of a gorilla's bed on several occasions, and could see that the male had seated himself with his back against a tree-trunk." He also says: "In both male and female the hair is found worn off the back; but this is only found in very old females. This is occasioned, I suppose, by their resting at night against trees, at whose base they sleep." The gorilla has only very partially acquired the erect position, and probably sits but little in the attitude common to man. In man the case is different; in proportion as his progenitors grew more and more erect, he must have lain less and less upon his stomach, and more and more upon his back or sides, and this is seen in the savage man during his lazy hours--who stretches himself on the ground in the sun, with his back propped, where possible, by a slight mound or the wall of his hut. The continual friction of the surface of the back would arrest the growth of hair; for hair grows where there is normally less friction, and vice versâ.

As man became more and more hairless, especially among savage and naked races, we should conclude that such a modification would be considered a beauty, and women would select such men in preference to more hairy individuals. The New Zealand proverb is: "There is no woman for a hairy man." Sexual selection, then, would play a very important part; and the difficulty of understanding how man became divested of hair is readily explained.

Haeckel says: "Even if we knew absolutely nothing of the other phenomena of development, we should be obliged to believe in the truth of the theory of descent, solely on the ground of the existence of rudimentary organs."

REPRODUCTION BY MEANS OF EGGS.

It might be thought there existed a missing link between animals which lay eggs and those which do not; this, however, is done away with in many instances--one, for example, is found in our commonest indigenous snake. The ringed snake lays eggs which require three weeks time to develop; but when it is kept in captivity, and no sand is strewn in the cage, it does not lay eggs, but retains them until the young ones are developed. This only shows how powerfully influences affect the habit of animals.

DOUBLE-SEXED INDIVIDUALS.

Another difficulty might be supposed to arise between animals which produce themselves other than by sexual reproduction. This has already been slightly touched upon; and it has been shown that numerous plants and animals propagate themselves through their double-sexed organs. It occurs in a great majority of plants, but only in a minority of animals; for example, the garden-snail, leeches, earth-worms, and many other worms. Every garden-snail produces in one part of its sexual gland eggs, and in another part sperm.

Parthenogenesis offers an interesting form of transition from sexual reproduction to the non-sexual formation of germ-cells (which most resembles it). It has been demonstrated to occur in many cases among insects, especially by Seebold's excellent investigations. Among the common bees, a male individual (a drone) arises out of the eggs of the queen, if the eggs have not been fructified; a female (a queen or working bee), if the egg has been fructified.

Gonochorismus or sexual separation, which characterizes the more complicated of the two kinds of sexual reproduction, has evidently been developed from the condition of hermaphroditism at a late period of the organic history of the world. In this case the female individual in both animal and plant produces eggs or egg-cells. In animals, the male individual secretes the fructifying sperm (sperma); in plants, the corpuscles, which correspond to the sperm.

INHERITANCE.

The remarkable facts of inheritance, extending to the reproduction of unimportant peculiarities of parts or organs (rudimentary parts) mentioned above, and the occasional outbreak of ancestral characters that have been dormant through several generations (some of which I will mention further on), might be thought perfectly unexplainable; but they are readily accounted for by the supposition that each part of an organism contributes its constituent and effective molecules to the germ and sperm particles. Mr. Sorby made numerous investigations with relation to the number of molecules in the germinal matter of eggs, and the spermatic matter supplied by the male. Omitting the alkali, Mr. Sorby takes the formula, C{72}H{112}N{18}SO{22}, as representing the composition of albumen. In a 1/2000 of an inch cube, he reckons--

Albumen 18,000,000,000,000 molecules. Water 992,000,000,000,000 " -------------------------------- 1,010,000,000,000,000 molecules.

Or, in a sphere of the same diameter, 530,000,000,000,000 of the two components. Taking a single mammalian spermatozoon, having a mean diameter of 1/6000 of an inch; "it might contain two and a half million of such gemmules. If these were lost, destroyed, or fully developed at the rate of one in each second, this number would be exhausted in about one month; but since a number of spermatozoa appears to be necessary to produce perfect fertilization, it is quite easy to understand that the number of gemmules introduced into the ovum may be so great that the influence of the male parent may be very marked, even after having been, as regards particular character, apparently dormant for many years." The germinal vesicle of a mammalian ovum being about 1/1000 of an inch, mean diameter, might contain five hundred million of gemmules, which, if used up at the rate of one per second, would last more than seventeen years. If the whole ovum, about 1/150 in diameter, were all gemmules, the number would be sufficient to last, at this rate, one per second for 5,600 years! This, however, is not probable; but Mr. Sorby's remarks has completely removed all doubt as to its physical possibility from the Darwinian theory; "and they prompt us," says Slack, "to a wonderful conception of the powers residing in minute quantities of matter."

The laws of inheritance are divisible into two series, conservative and progressive transmission; the laws of adaptation to direct (active) or indirect (potential) adaptation.

External causes often influence the reproductive system, especially in organism propagating in a sexual way. This can be strikingly shown in artificially produced monstrosities. Monstrosities can be produced by subjecting the parental organism to certain extraordinary conditions of life; and curiously enough, such an extraordinary condition of life does not produce a change of the organism itself, but a change in its descendants. The new formation exists in the parental organism only as a possibility (potential); in the descendants it becomes a reality (actual). Most commonly, monstrosities with very abnormal forms are sterile, but there are instances where they reproduce their kind and become a species. Geoffroy St. Hilaire, who perhaps made the deepest investigations ever conducted into the nature and causes of their production, first conceived the idea of artificially producing them, and to this end he began modifications of the physical conditions of the evolution of the chicken during natural and artificial incubation. He determined the fact that monsters could be produced in this way, but scarcely carried his investigation further. This work has been taken up by M. Dareste, and he has lately published a volume in Paris which recounts the results of a quarter of a century's experimenting. Eggs, he states, were submitted to incubation in a vertical instead of a horizontal position; they were covered with varnish in certain places so as to stop or modify evaporation and respiration. The evolution of the chick was rendered slower by a temperature below that of the normal heat of incubation. Finally, eggs were warmed only at one point, so that the young animal, during development, was submitted at different parts to variable temperatures.

These perturbations resulted in the most curious and unlooked for deformities in the embryo, some being not alone peculiar to the bird, but being similar to those which have been recognized in many other animals, and even in the human species. The data obtained have been deemed so important that M. Dareste has recently received the Lacaze prize for physiology from the French Academy of Sciences.

It would be impossible to review even a fraction of the many forms of monstrosities which M. Dareste has discovered. Those that we give will, however, suffice to convey an idea of the wonderful variations produced. Fig. 1 is a chick embryo with the encephalon entirely outside the head, the heart, liver, and gizzard outside the umbilical opening, right wing lifted up beside the head, and the development of the left one stopped. In Fig. 2 the encephalon is herniated and marked with blood spots, the eye is rudimentary and replaced by a spot of pigment, the upper beak is shorter than the lower one, while the heart, liver, etc., are all outside. In Figs. 3 and 4 the head is compressed, eyes well developed, but in the back instead of in the sides of the head; the body is bent, abdominal intestines not closed, heart largely developed and herniated. The literal references to the foregoing are: am, amnion; al, allantois; v, vitellus; h, encephalon; i, eye; c, heart; f, liver; g, gizzard; ms, upper, and mi, lower member.

The commonest case of monstrosity observed by M. Dareste has been that of the head protruding from the navel, and the heart or hearts above the head. This is a most extraordinary and new monster, and, if it persist, a chicken with its heart on its back, like a hump, may be expected. A curious fact discovered is the duplicity of the heart at the beginning of incubation, two hearts, beating separately, being clearly seen. Another anomaly consists in heads with a frontal swelling, which is filled by the cerebral hemispheres.

M. Dareste's artificial monsters are all produced from the single germ or cicatricule (as the white circular spot seen in the yellow of the egg, and from which the embryo springs, is termed). He has not yet been able to determine artificially the production of monsters, the origin of which takes place in a peculiar state of the cicatricule before incubation. But having submitted to incubation some 10,000 eggs, he has obtained several remarkable examples of double monstrosities in process of formation, some representations of which are given herewith. Fig. 5 shows three embryos, all derived from a single cicatricule. Fig. 6 represents three embryos from two cicatricules. On one side of the line of junction are two imperfectly developed embryos, one having no heart. The single embryo on the other side is generally normal, but has a heart on the right side. In Fig. 7 are twins, one well formed, the heart circulating colorless blood, the other having no heart and a rudimentary head. Fig. 8 exhibits a double monster with lateral union. The heads are separate, and there are three upper and three lower members, those of the latter on the median line belonging equally to each of the pair.

ACQUIRED QUALITIES.