Part 4 (1/2)

Cuvier, Haller and Leibnitz adopted substantially these views. The latter found them to support his opinion that everything was the result of growth from monads, and that there was no such thing in all nature as generation.

Such a theory was very simple, but it explained nothing except the bare production of offspring. It gave no clue to their endless variations, nor to the fact that they often resembled the father more than the mother. According to this theory the offspring should resemble the mother, as the complete individual is formed by her and should be in her image.

Leeuwenhock, one of the early microscopists, by the aid of his lenses, opened a new world to mankind, and discovered the sperm cells to be active, living, moving elements, and he gave a death-blow to the belief that the perfect organism exists in the ovum; but he went to the opposite extreme, and maintained that it exists in the male cell and that it is only fed and developed by the female. Even today we find in a vague way both these theories held by educated persons.

We are indebted to Harvey in the early part of the eighteenth century for advocating the view held by Aristotle, now known as _Epigenesis_, and combatting the view of growth from a miniature, but already perfectly formed animal, to a visible one. Epigenesis consists in the successive differentiation from the relatively h.o.m.ogeneous elements as found in the egg, to the complicated parts and structure as seen in the offspring.

According to Huxley, this work of Harvey alone would have ent.i.tled him to recognition as one of the founders of biological science, had he not immortalized himself as the discoverer of the circulation of the blood.

Not long after Harvey's publication, Casper Frederick Wolf established the theory of epigenesis upon a firm foundation, where it still remains.

The doctrine of _epigenesis_ has very much complicated the whole question of heredity. No wonder even so great a mind as that of Darwin exclaimed, ”The whole subject is wonderful.” How can an egg, which in structure is comparatively simple, an aggregation of cells, not one of which bears the slightest resemblance to any organ in the body, develop into the perfect individual? How can this egg, formed in special organs, develop other organs than those like the ones in which it was formed?

How can s.e.xual cells develop brain cells, with their wonderful modes of action?

We cannot explain the philosophy of heredity without being able to answer these questions; but difficult as is the problem, our biologists have made various attempts at an explanation. I cannot go over all the various speculations, but only those most intimately connected with the subject will be mentioned.

The first is Darwin's own attempt at an explanation by the theory of _pangenesis_, or genesis from every part. He saw the necessity of having in the s.e.xual cells some power or force to represent the other organs and functions of the body, else how could these organs be formed in the embryo? Pangenesis was supposed to be accomplished as follows: Every organ through its cells gives off _gemmules_. These are inconceivably small, too small for any microscopical vision; also inconceivably great in numbers, and with great power of growth and multiplication. They pa.s.s from the various organs in which they are formed to the special s.e.x organs for generating the s.e.xual cells; some of them are stored up as representatives of the various organs from which they have been given off. The consequence is that every egg has in it something from every organ in the body of both parents which is able, during gestation, to develop into that organ.

According to this theory, for instance, if no gemmules are given off from the brain, then no brain can be developed from the egg, and so of other organs. As in a representative government, all parts of the country send representatives to the capitol to do the bidding of the people, so every organ of the body sends representatives to the s.e.xual cells to form their respective organs; without them these organs would not be formed.

There are many objections to pangenesis, but they need not be named here. It occurred to Galton, whose studies in heredity have been more prolific of good than those of any other man, to test it by practical experiment. If these gemmules are circulating in the blood of animals before being stored up in the s.e.xual cells, by transfusing blood from one variety of any species to another it ought to affect the offspring of this other. For his test cases he chose eighteen silvergrey rabbits which breed true, and into their bodies he transfused the blood of other different varieties, in several cases replacing one-half of this fluid.

There were eighty-six offspring bred at once from these silvergrey rabbits, and all true silvergreys. The theory did not work. But if it did not work in practice, it certainly worked on the intellects of biologists everywhere, exactly what Darwin wished; it set them to thinking. It acted as a ferment, so to say, and brought forth a rich harvest in speculation if not in actual knowledge.[106:A]

CONTINUITY OF THE GERM-PLASM.--The only other theory which I shall mention is that of Weismann, which has been before the public for more than a decade, and it is safe to say it has produced a more profound impression upon biologists than all others. It has its basis in what he calls _continuity of the germ-plasm_. By the germ-plasm is meant that part of the germ cell containing all the chemical and physical properties, including the molecular structure, which enables it to become, under appropriate conditions, a new individual of the same species as the parents. In it lies hidden all the characteristics both of the species and of the future individual. In it lies all the phenomena of heredity. It is the product of the coalescence of the male and female elements requisite for reproduction. Only, however, in the nuclear substance is to be found the hereditary tendencies. Now, this germ-plasm is _continuous_, that is to say, it contains not only material from both parents, but from grandparents and greatgrandparents, and so on indefinitely. This germ-plasm is exceedingly minute in quant.i.ty, but has great power of growth. Not all is used up in the production of any individual, but some is left over and stored up for the next generation. The germ-plasm might be represented as a long creeping root, from which arise at intervals all the individuals of successive generations. The amount of ancestral germ-plasm in each fertilized ovum is calculated in the same way that stock breeders calculate the amount of blood of any ancestor running in any individual.

For instance: The germ-plasm contributed by the father and mother is each one-half; each grandparent one fourth, and so on. Ten generations back each ancestor contributes only one part in one thousand and twenty-four parts. This continuity has by some been called the immortality of the germ-plasm. Theoretically, the original Adam and Eve have contributed an infinitesimal part. This probably explains why there is so much of the original Adam in most of us. By it we are able to explain that wonderful fact of _atavism_, or the appearance of characters from a remote ancestor in offspring. Some of the germ-plasm from this ancestor by some means has had an opportunity to grow rapidly and contribute more than its share in the production of the individual in which it appears.

It also enables us to explain the fact that no two individuals are quite alike, but that there is constant variation. Each person is the product of a mult.i.tude of ancestors, and the germ-plasm which produced them is never mixed, in quite the same proportion, nor do the different parts grow with quite the same vigor.

It was on this theory of the continuity of the germ-plasm that Weismann built his doctrine of the non-transmission of acquired characters. On this subject he says: ”Hence it follows that the transmission of acquired characters is an impossibility, for if the germ-plasm is not formed anew in each individual, but is derived from that which preceded it, its structure, and above all, its molecular const.i.tution, cannot depend upon the individual in which it happens to occur, but such an individual only forms, as it were, the nutritive soil at the expense of which it grows, while the latter possessed its character from the beginning, that is, before the commencement of growth.” Of this, however, I will speak later.

A RATIONAL VIEW OF HEREDITY.--I might continue giving other theories of heredity--Haeckel's, for instance--or the metaphysical theory, but it is hardly necessary. I do not accept in full any of them. Their authors, it seems to me, have not worked along the lines of evolution, but have gone further than was necessary into the fields of speculation. Darwin, in his theory of Pangenesis, admitted this frankly, and yet he clung to the idea with great tenacity. If we take the unicellular organisms which multiply by division, we may see that heredity is simple. One unicellular individual growing larger than is convenient, divides into two. Each is like the other. It could hardly be different. Reproduction by spores or buds is practically the same thing. The spores or buds are minute particles of the parent organism. When it comes to the coalescence of the germ and sperm elements from two organisms, the phenomena become more complicated, and it is still more so as the animal rises in the scale of creation; but I believe the processes of organic evolution have gone on so slowly that the s.e.xual cells have acquired the power to transmit the whole organism without the necessity of the germ-plasm being continued from parent to offspring indefinitely, and also without the aid of pangenesis.

The egg has acquired a tendency to develop in a certain direction. Just how we cannot tell, further than to say that it was probably the result of variation first and natural selection selecting out those variations most suitable. It is this tendency to vary that gives rise to many of the phenomena of heredity. The subject is, for the present, beyond our power to settle satisfactorily, and so hypotheses must be resorted to.

The s.e.xual cells, comparatively simple in anatomical structure, must be highly complex in their molecular structure; and the more highly evolved the organism, the more complex becomes this molecular structure. If it were possible to study this molecular structure we should be able to understand the whole subject far better than is possible now. But this is not possible, and there is little hope that we shall ever be able to accomplish it.

HEREDITY AND THE EDUCATION OF CHILDREN.--The next question which comes up for consideration is that of the education of children and its relation to heredity. This brings us at once to the problem as to whether acquired characters are transmitted to offspring or not. If acquired characters are transmitted, the relation of heredity to education must be very close and important. If acquired characters are not inherited, then heredity and education have a very different relation. That acquired characters are transmitted has long been believed. It was the belief of Lamarck. He tried to explain the structure of the organism by this principle. The ill.u.s.tration of the long neck of the giraffe is familiar to every one. It originated by the constant stretching of this part to obtain food from the trees. In times of scarcity, he had to exert himself in this way still more to reach the higher branches. The young of the giraffe had longer necks than their parents because of the efforts of the latter in this way. So the keen sight of birds, it was argued, was acquired in the same manner. The hawk had to exercise his eyes most vigorously to discern his prey at a distance, and his offspring inherited this keenness of sight acquired by the exercise of his ancestors.

Darwin believed that the effects of the exercise of any part were transmitted. He says: ”We may feel a.s.sured that the inherited effects of the use and disuse of parts will have done much in the same direction with natural selection in modifying man's structure of body.”

We may say that this belief has been held by the common people, uneducated in science. They not unfrequently get at truths in a rude way long before the scientists do. Many parents tell us their children are strongly influenced by some particular occupation of the mother during pregnancy. So strong is this belief, that many mothers are in our times trying to influence the character of their unborn children by special modes of life, by cultivating music or art, or science, in order to give the child a love for these pursuits.

It is by Herbert Spencer that this has been most ably presented. Indeed, he holds that there is no explanation of evolution without the transmission of the effects of the use and disuse of parts. His words are: ”If there has been no transmission of acquired character there has been no evolution.”

He also says: ”If we go back to the genesis of the human type from some lower type of primates, we see that while the little toe has ceased to be of any use for climbing purposes, it has not come into any considerable use for walking or running. It is manifest that the great toes have been immensely developed since there took place the change from arboreal to terrestrial habits. A study of the mechanism of walking shows why this has happened. Stability requires that the line of direction--the vertical line, let fall from the center of gravity--shall fall within the base, and the walking shall be brought at each step within the area of support, or so near that any tendency to fall may be checked at the next step. A necessary result is that _if_ at each step the chief stress of support is thrown on the outer side of the foot, the body must be swayed so that the line of direction may fall within the outside of the foot, or close to it; and when the next step is taken it must be similarly swayed in an opposite direction, so that the outer side of the foot may bear the weight. That is to say, the body must oscillate from side to side, or waddle. The movement of the duck when walking shows what happens when the points of support are far apart.

This kind of movement conflicts with efficient locomotion. There is a waste of muscular energy in making these lateral movements, and they are at variance with the forward movement. We may infer, then, that the developing man profited by throwing the stress as much as possible on the inner side of the feet, and was especially led to do this when going fast, which enabled him to abridge the oscillations, as indeed we see it now in the drunken man. Then there was thrown a continually increasing stress upon the inner digits as they progressively developed from the efforts of use, until now the inner digits, so large compared with the outer, bear the greater part of the weight, and being relatively near one another render needless any swaying of the body from side to side in walking. But what has meanwhile happened to the outer digits? Evidently as fast as the great toes have come more and more into play and the small ones have gone more and more out of play, dwindling for--how long shall we say?--perhaps 100,000 years.” In other and simpler words, the great toe of man has wonderfully developed since he began to walk upright. This has been from greater use, and the transmission of the effects of this use to offspring. The small toe has decreased in size proportionately. This we can reasonably infer has been the result of disuse, the effects of which were also transmitted to offspring.

A still more remarkable ill.u.s.tration of the effects of use and disuse is seen in the sense of touch in different parts of the body. Prof. Weber, in his laboratory for experimental psychology, has worked out this difference most minutely. He finds that by taking a pair of compa.s.ses, the points of which are less than one-twelfth of an inch apart, the end of the forefinger is not able to distinguish more than one point. Going to the middle of the back we have the least discriminating power in the skin, for the points must be separated two and one half inches before the nerves can decide that there are two. Any one may test this on himself. Between these extremes we have many differences. The end of the nose has four times as great power of discrimination as the forehead. When we come to the tip of the tongue, we find it far excels any part of the body in its power of tactual discrimination, it being twice that of the forefinger. In every case we find there is greatest delicacy of touch in those parts where this sense has been most exercised. The tongue is being constantly exercised on our food, on the roof of the mouth, the teeth, etc. It is rarely idle. There is in man no advantage for his survival, Mr. Spencer a.s.serts, by having such a sensitive tongue. He could get on just as well without it. He regards it as a case where the exercise of a function has exalted it remarkably, and this exaltation has been transmitted to offspring. Natural selection, he thinks, is not sufficient to account for it. Natural selection only preserves those characters which will give their possessor some advantage in the struggle for existence.