One can not, however, establish a general law upon the study of a highly specialized race of animals such as the shell-bearing cephalopods, and we must search through the entire animal kingdom to thoroughly test Hyatt's hypothesis; and zoologists have not yet done this, for the paramount interest in studies of heredity now centers around Mendel's law. Yet Hyatt has raised a burning question—is the course of evolution a predetermined thing, and do the growth-stages of the individual reveal to us the past, present and future of its race? Hyatt says they do, for he states that organisms tend to produce offspring varying in a certain well-defined direction so that we may indicate with tolerable certainty what species a given form can or might produce.
It is sad to think that so few young men have followed him into this great field of study, for a student's life is not wasted even if after years of labor he discovers that his preconceived hypotheses were false and he can not fathom nature's secrets, for it is not for science to advocate, but only to search, hoping to discover.
We will now take up the discussion of Hyatt's law of acceleration or tachygenesis as he finally called it; although it was commonly known as the "old age theory." According to Hyatt, modifications, once they appear, tend to develop in successive generations at earlier and earlier stages of growth, so that modifications which first appear in adult life or even in old age before the animal becomes sterile will afterwards be developed in the young stages of descendants. Finally, indeed, they appear in the embryos or are crowded out and replaced by later characters, Hyatt believed this law of acceleration to be an invariable mode of action of heredity.
He also believed in the inheritance of acquired characters, and held that the organism is plastic and irritable and responds to external stimuli by internal reactions which manifest themselves as hereditary modifications of structure. It is interesting to see that the recent researches of Tower and MacDougal have shown that artificially produced changes in the environment may affect the germ-cells and produce hereditary modification of structure.
Hyatt maintained that the evolution of new forms has been more rapid than is generally supposed, and in this he has been supported by the classic work of DeVries, who shows how suddenly a new form may appear and maintain itself. It also accords with Bateson's demonstrations of the frequency of "discontinuous variations." As Farlow says: "Our so-called species are merely snap-shots at the procession of nature as it passes along before us."
Hyatt also states that the development of ancestral forms is usually simple and direct; that of their more specialized descendants becomes gradually indirect with complicated larval or intermediate stages; and that of the terminal retrogressive stages, before extinction becomes again more or less direct. Thus the last is like the first.