188 THE ENOLUTION THEORY 



But that tlie c'Oiuploto disiipiH'ariiW'o ol' (ho ilott>nninnnts only ooiwivs 

 about Avith oxtrouio slowiu'ss, so that \vhoK> o-oolngu'al jioriods aro 

 soniotiuies not ouough for its aceoraplishiuent, wo have alvoaity learut 

 Itoui our study of rudimoutary organs, instanoos of whicli can ln' 

 demonstrated in ovory higher animal, hearing witness to the 

 presence of the relevant oi'gans or structures in the ancestiirs ol" 

 the species. 



We can infer with certainty, from the obser^•ational data at 

 our disposal, that the disajipivvrance of useless parts is ri>gulated 

 by definite laws ; but it is too soon to attempt io formulate these 

 laws, or even to trace them back to their mechanical causes. As we 

 have already said, a nuich more comiirehensive collection of facts, and 

 above all one which lias been made on a definite ]ilan, is a necessary 

 prelinnnary 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, luid is transferred gradually backwards into the 

 embryogenesis. Thus the two lingers of birds which havi- disappeannl 

 since Crt'taceous times are still indicated in every bird-embryi), 

 though they sulisequently degenerate. In \arious manunals ' pre- 

 lacteal tooth-germs' hax'e been demonstrated in the jaws of embryos, 

 which s1k)w us that not only did ancestors exist whose dentition was 

 the modern ' nulk-teeth,' but that still more remote ancestors possessed 

 another set of teeth, which was crowded out by the ' niilk-leetli ' ; 

 thus the teeth of the ancestors of the modern right whale (Bakviut 

 'niyt:ticetuti) are only i-i'preseuted in the endiryo of to-day in the I'orm 

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

 istic of the wrist of lower vertebrates only appears in Man at a x'cry 

 early embryonic stage, and disappears again as such in the I'urther 

 course of the endnyogenesis. 



W(^ may perhaps give a jireliminary statement ol' this law as 

 follows: It is impossible that any part or oi'gan should bo removed 

 suddenly from the ontogeny without bringing the whole into disorder, 

 and the lea.st serious distin-bance oL' the course of di'\'olopment will 

 undoubtedly be caused if the tinal stage of the part in qu(>stion 

 become rudimentary lirst. Only after this has happened, and the 

 neighbouring parts ha\e adapted themselws to the di.sappearanci', 

 can this extend to the stages innnediat(ily preceding it, so that these 

 too degen(M-ate, and allow the siuTonnding parts to adapt themselves. 

 The further back into the ontogeny the disappearance extends the 

 grc^ati'r will be the nundior of other structures aJfi'ctod in some way 

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

 sudd(Mdy into new conditions, else the whole cour.se of development 



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