No. I.] OSTEOLOGY OF PCEBROTHERIUM. 7 



might be regarded as being re-acquired, and therefore atavistic. 

 If none of these alternatives can be accepted, it only remains 

 to assume that the accessory molar has been added dc novo. A 

 somewhat similar case is that of the llama with four lower 

 molars recorded by Riitimeyer (No. 25, p. 55). 



(7) In cases of the reduction of parts, is such reduction always 

 accomplished in the same manner ? For example, Fiirbringer 

 maintains that the reduction of vertebrae must always proceed 

 from the hinder end of the column, and that interpolation or 

 reduction of vertebrae in the median or anterior region does not 

 take place (No. 11, p. 328). Ihering, on the contrary, expressly 

 states that such interpolation and suppression does take place 

 (No. 15), a view which is adopted by Parker on the basis of his 

 studies of the development of Chelone viridis : " In the adult I 

 can find only 41 developed vertebrae ; viz. 8 cervical, 10 dorsals, 

 2 sacral, 21 caudal. But in the third and fourth stages there are 

 at least 15 somatomes in the cervical region ; in the dorso-lum- 

 bar-sacral, 12 (as in the adult) ; and 24 in the caudal — 51 in all. 

 Thus we miss in the adult 7 in the cervical and 3 in the caudal 

 — 10 in all " (No. 23, p. 47). 



(8) Besides the questions of a more general nature already 

 mentioned, there are many obvious ones of a special character, 

 of which we may select as an example the following : What 

 are the effects which follow as a mechanical necessity from great 

 increase or diminution of the size of the body ? 



This list of " open questions " with regard to the mode in 

 which evolution operates in any given case might, of course, be 

 greatly extended ; but for the purposes of the present investiga- 

 tion it will suffice. At first sight it might seem that the ques- 

 tions here propounded were unanswerable, because the solution 

 of them must be drawn from the careful study of phyletic 

 series, while the proper construction of the latter depends upon 

 a correct understanding of the method of development. For- 

 tunately, however, certain series of extinct mammals offer us a 

 way of escape from this "vicious circle," because they present 

 us a series of forms which we have every reason to believe stand 

 to each other in the relation of ancestor and descendant, a rela- 

 tionship which may be verified by tracing the changes of form 

 and function through the minutest gradations back to starting- 

 points very different from the final results. Furthermore, in 



