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 sexual 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 pass from the various organs in which they are formed to the special sex organs for generating the sexual 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 sexual 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 sexual 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 quantity, 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 multitude 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 constitution, 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—Hæckel'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 sexual 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 sexual 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 illustration 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.
