The Principles of Heredity With Some Applications

portant for a right understanding of what I conceive to be the true principles of heredity that it will be well to sum up and emphasize their main contentions briefly. Since every individual follows (with variations) in the developmental footsteps of his predecessors, the recapitulation (with inaccuracies corresponding to ancestral variations) of the life-history of his race is inevitable in his own development?unless indeed he happens to be an extraordinary and incredible sort of monster. No such monsters can have occurred in his ancestry, for monsters, lacking adaptation to the environment, invariably perish. While it is obviously impossible for any individual to so vary from his immediate predecessors as to recapitulate the later stages of development without first substantially recapitulating the earlier stages, it is of course quite possible for the converse to happen. Thus a human being cannot become an adult without first passing through the embryonic and foetal stages; but it is quite possible for him to cease developing at any point. When this happens, when there is an omission of the later stages, we have an instance of reversion, of regression. It is customary to regard reversion as due to the sudden activity of hitherto " latent ancestral units," but it is infinitely more probable that it is due to incomplete recapitulation. Since organisms vary in all possible directions, there is no conceivable reason why they should not vary in this particular di...