174 BIOLOGICAL LECTURES. 



the correct interpretation of the results of experiment is often 

 fraught with such difficulty, that every possible aid from other 

 sources must be utilized. 



Still developmental mechanics will be of more or less service 

 to these morphological disciplines in return for what it is contin- 

 ually receiving from them. 



It will open the eyes of the descriptive observer to many 

 structural relations hitherto overlooked ; structures which have 

 been scarcely appreciated will acquire a deeper significance ; 

 many a problem arising from descriptive study and incapable 

 of solution through observation on normal phenomena will be 

 elucidated, and the causal deductions of these sciences will be 

 corrected or established on a firmer basis. Thus the doctrine 

 of the transposition of cells during embryonic formation — a 

 doctrine which has of late been greatly expanded by His — 

 will be proved to be correct only by experiment, and tested as 

 to the extent to which it is claimed to obtain, and traced back 

 to its causes. In like manner our ideas derived from com- 

 parison of different phases of cell-division require direct experi- 

 mental proof, or confirmation and extension with respect to 

 the immediate causal interrelations of these processes. It was 

 only through causal observation that life was infused into the 

 dead facts of corrosion anatomy, when the laws wJiicJi govern 

 the ramification of blood-vessels were discovered. 



Comparative anatomy will be able to receive a great deal of 

 assistance from developmental mechanics, especially in extend- 

 ing the problems with which it deals. As comparative anatomy 

 endeavors to ascertain the genetic connection, the " Stamm- 

 baum" of organisms, it is itself essentially a causal science. 

 It analyzes structures into the two components, variation and 

 heredity. It is true that both of these, as understood in com- 

 parative anatomy, are general formative principles, but, in the 

 first place, they are of much greater diversity than the com- 

 plex components given above as illustrations, and in the second 

 place, they are not uniform, i.e., not always constant in their 

 modes of operating. 



Heredity is a constant principle, always operating in the same 

 way, only in so far as it depends, according to Weismann and 



