SOCIAL EVOLUTION I VARIATION AND HEREDITY We know that among both animals and plants the offspring tend to resemble their parents. The young of a dog is always a dog and never a cat. Rabbits do not give birth to guinea pigs. Roses will not grow from potato seeds. Each kind of animal and plant breeds true as the biologist says. But this principle has not always been recognized. The Greeks believed that certain animals originated from plants. In the Middle Ages it was believed that the goose-barnacle (a kind of crustacean) transforms into a barnacle-goose. But we have accumulated sufficient experience of nature in the centuries that have passed since Mediæval times to affirm with confidence that animals and plants do breed true. It is a fact perfectly familiar to breeders that the young usually resemble the particular individuals from which they have sprung. Therefore we can say that not only do plants and animals, when they reproduce, give rise to young which belong to the same species as their parents, but further than this they tend to resemble their parents in individual peculiarities. Thus, among domestic cattle, the offspring resemble the parents in such qualities as size, form, color, amount and quality of milk, as well as many other qualities.1 1 Metcalf, M. M.-Organic Evolution, 3rd ed., 1911, p. 6. Although the offspring tends to resemble the parent, it is evident that the resemblance is not an exact one. The resemblance between the different members of the family is such that brothers and sisters are quite distinguishable. Each has his or her own individual peculiarities. No two are exactly alike. These facts of individual difference we group under one term. We call this phenomenon variation. We can now change our statement and say: under the influence of heredity the young tend to resemble their parents, but this resemblance is never exact. It is more or less imperfect. If one doubts this fact of variation let him take a thousand individuals of any species with regard to any single character and he will be convinced. If one measure the lengths of a thousand oak leaves taken from the same tree, he will find that some are considerably longer than others, but that within certain limits most of the leaves have approximately the same length. So it is with any character of any animal or plant, there is much variation. The winter birds of east Florida show a variation in size of from fifteen to twenty per cent. among specimens of the same species and sex when taken in the same locality.2 The relation then, between parents and offspring, is such that like tends to beget like, while at the same time opportunity is afforded for the individual new departures which we have called variations. But how does it happen that like tends to beget like? Why is it that the young of a horse will always be a horse and not a zebra? How is it that nature keeps things in order? For some time it has been known by biologists that when the parents' body is developing from the fertilized ovum, a resi2 Ibid., p. 9. due of unaltered germinal material is kept apart to form the reproductive cells, one of which may become the starting point of a child. On this point Galton has written, "The total heritage of each man must include a greater variety of material than was utilized in forming his personal structure. The existence in some form of an unused portion is proven by his power. . . of transmitting ancestral characters that he did not personally exhibit. Therefore the organized structure of each individual should be viewed as the fulfilment of only one out of an indefinite number of mutually exclusive possibilities. His structure is the coherent and more or less stable development of what is no more than an imperfect sample of a large variety of elements."3 The idea was more independently expressed and more fully developed. by Weismann in 1893. It is now the basis of our explanation of why like tends to beget like. It is the theory of the continuity of germinal plasm. Weismann says, "In development a part of the germ-plasm (i. e., the essential germ material) contained in the parent egg-cell is not used up in the construction of the body of the offspring, but is reserved unchanged for the formation of the germcells of the following generation." Thus it has been said that the parent is rather the trustee of the germplasm than the producer of the child. The philosopher Bergson has said, "Life is like a current passing from germ to germ through the medium of a developed organism. The essential thing is the continuous progress indefinitely pursued, an invisible progress, on which each visible organism rides during the short interval of time given it to live." The reason why like tends to 3 Galton, F.-Natural Inheritance, 1889, p. 18. 4 Thomson, J. A., & Geddes, P.-Evolution, 1911, p. 114. |