EMBRYOGENY 213 



also that there are many vacillations in the relations 

 of the intra- to the extra-uterine period of development. 



In the Alpine Salamanders we observe how an 

 internal stage can be omitted ; in the Mountain Lizard 

 the contrary can be effected by experiment. 



(c) We will now assume that the causes which have 

 led to the said change in the Alpine Salamander con- 

 tinued, and also increased in power, so that the inutility 

 of a constructed gill stage became ever greater. We 

 know that in such cases a tendency immediately shows 

 itself and this is the case with all organisms no longer 

 to form such non-functional organs. The result will 

 be that at first a defective construction follows, until 

 finally the former organ perhaps entirely disappears, 

 or it may be used in quite another form. The germ 

 development leads ever more directly to the new form, 

 since the previous one, under the altered circumstances, 

 is no longer suited to its purpose. 



The gill branchse of the larval Alpine Salamander 

 could consequently quite well be so far reduced finally 

 that only splits or folds would be formed in the gullet 

 as, with the other Salamander larvae and gill-breathing 

 animals, precede the formation of the gills as preparatory 

 stages. Then we should have a rudiment in the sense 

 of a formation which, by its construction and position 

 in the organism, has a similarity to definite organs of 

 other animals, but so imperfectly developed, or even 

 only suggested, that they can no longer exercise a func- 

 tion, or at any rate only extremely imperfectly. That 

 would be specially a rudiment of an evolutionary stage. 



