188 THE EVOLUTION THEORY 



But that the complete disappearance of the determinants only comes 

 about with extreme slowness, so that whole geological periods are 

 sometimes not enough for its accomplishment, we have already learnt 

 from our study of rudimentary organs, instances of which can be 

 demonstrated in every higher animal, bearing witness to the 

 presence of the relevant organs or structures in the ancestors of 

 the species. 



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 (Balcemt 

 Qiiysticetus) are only represented in tlie embryo of to-day in the form 

 of dental pits. And, as we saw already, the Os centrale so character- 

 istic 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 afiected in some way 

 or other by the degeneration, and these must not all be brought 

 suddenly into new conditions, else the Avhole course of development 



