On tlte Mechanical Conception of Nature. 703 



this theory it would be an unavoidable postulate, 

 that by such a process of per saltum development 

 there arises not merely a new type of some 

 species, but at the same time individuals capable 

 of living and of persisting under, and fitted to, 

 given conditions of life. But every naturalist who 

 has attempted to completely explain the relation 

 between structure and mode of life knows that 

 even the s-mall differences which separate one 

 species from another, always comprise a number 

 of minute structural deviations which are related 

 to well defined conditions of life he knows that 

 in every species of animal the whole structure is 

 adapted in the most exact manner in every detail 

 to special conditions of life. It is not an ex- 

 aggeration when I say in every detail, since the 

 so-called " purely morphological parts " could not 

 be other than they are without causing changes in 

 other parts which exercise a definite function. 1 

 will not indeed assert that in the most closely 

 related species all the parts of the body must in 

 some manner differ from one another, if only to a 

 small extent; it seems to me not improbable, 

 however, that an exact comparison would very 

 frequently give this result. That animals which 

 are so widely removed in their morphological 

 relations as Medusae and Polypes, or Trematoda 

 and their " nurses," are differently constructed 

 in each of their parts can, however, be stated 

 with certaintv 



