DEVELOPMENTAL MECHANICS. 



175 



others, on the continuity and variations of the germ-plasm, and 

 hence on assimilation. When we are dealing besides with the 

 inheritance of somatogenic, or so-called acquired characters, 

 the same word is used to designate modi operandi of a totally 

 different nature. 



The concept variation (adaptation) comprises so many dif- 

 ferent operations that Haeckel has established for them a 

 whole series of "laws" (19). Both heredity and variation, 

 however, are in urgent need of causal explanation, i.e., of 

 analysis into their uniformly operant components. This anal- 

 ysis is one of the tasks of developmental mechanics. This 

 is true also of ccenogenesis w\& of the so-called "fundamental law 

 of biogenesis." 



The hypotheses which comparative anatomy, like every other 

 science, continually employs, have essentially the character of 

 developmental mechanics. 



As this fact does not seem to be sufficiently well known, a 

 few illustrations may be adduced here. 



Gegenbaur rejects the homology of the ventral nerve-cord 

 with the spinal cord (20) mainly for the reason that he regards 

 the difference in the respective "positions" of the two organs as 

 much more important than the agreement of their occurrence 

 throughout the whole length of the animal, their metameric 

 segmentation, similarity of ramification, and composition of the 

 same form-elements. This opinion rests upon the assumption 

 that in phylogeny an organ may more easily arise anew and 

 independently of a preexisting organ with which it has in 

 common the same biological constituents, essentially the same 

 distribution, the same segmentation, and the same function, 

 than that the latter organ should have changed its position to 

 such an extent, viz., from the ventral to the dorsal side of 

 the animal. 1 



As will be seen, this assumption is purely one of develop- 

 mental mechanics and was certainly a bold hypothesis in the 

 state of developmental mechanics at that time ; and although 

 we do not doubt its truth in this particular case, Gegenbaur 



1 I have taken the liberty of correcting an obvious lapsus calami in this sen- 

 tence. W. M. W. 



