We can infer with certainty, from the observational data at our disposal, that the disappearance of useless parts is regulated by definite laws; but it is too soon to attempt to formulate these laws, or even to trace them back to their mechanical causes. As we have already said, a much more comprehensive collection of facts, and above all one which has been made on a definite plan, is a necessary preliminary condition to this. But so much at least we may gather from the facts before us, that the degeneration of an organ begins at the final stage, and is transferred gradually backwards into the embryogenesis. Thus the two fingers of birds which have disappeared since Cretaceous times are still indicated in every bird-embryo, though they subsequently degenerate. In various mammals 'pre-lacteal tooth-germs' have been demonstrated in the jaws of embryos, which show us that not only did ancestors exist whose dentition was the modern 'milk-teeth,' but that still more remote ancestors possessed another set of teeth, which was crowded out by the 'milk-teeth'; thus the teeth of the ancestors of the modern right whale (_Balæna mysticetus_) are only represented in the embryo of to-day in the form of dental pits. And, as we saw already, the Os centrale so characteristic of the wrist of lower vertebrates only appears in Man at a very early embryonic stage, and disappears again as such in the further course of the embryogenesis. We may perhaps give a preliminary statement of this law as follows: It is impossible that any part or organ should be removed suddenly from the ontogeny without bringing the whole into disorder, and the least serious disturbance of the course of development will undoubtedly be caused if the final stage of the part in question become rudimentary first. Only after this has happened, and the neighbouring parts have adapted themselves to the disappearance, can this extend to the stages immediately preceding it, so that these too degenerate, and allow the surrounding parts to adapt themselves. The further back into the ontogeny the disappearance extends the greater will be the number of other structures affected in some way or other by the degeneration, and these must not all be brought suddenly into new conditions, else the whole course of development would suffer. Thus at first only those determinants may disappear--and can disappear according to the laws of germinal selection--which control the final form of the useless organ, then those just preceding them, which controlled, let us say, its size, and thus more and more of the previously active determinants disappear, and hand in hand with this disappearance there is variation of all the parts correlated with the dwindling condition of the organ, so that their own development and that of the animal as a whole suffers no injury. If it were otherwise, if when a part became useless its collective determinants were all to disappear at the same time, the