PROGRESS NOT ALWAYS DIFFERENTIATION. 283 



velopment, has set forth the following proposition as one of 

 the principal laws in the ontogenesis of the animal body : — 

 " The degree of development (or perfecting) depends on 

 the stage of separation (or differentiation) of the parts." ^^ 

 Correct as this proposition may be on the whole, yet it is not 

 universally true. In many individual cases it can be proved 

 that divergence and progress by no means always coincide. 

 Every progress is not a differentiation, and every differenti- 

 ation is not a progress. 



Naturalists, guided by purely anatomical considerations, 

 had already set forth the law relating to progress in organ- 

 ization, that the perfecting of an organism certainly de- 

 pends, for the most part, upon the division of labour among 

 the individual organs and parts of the body, but that there 

 are also other organic transformations which determine a 

 progress in organization. One, in particular, which has 

 been generally recognized, is the numerical diminution of 

 identical parts. If, for example, we compare the lower 

 articulated animals of the crustacean group, which possess 

 numerous pairs of legs, with spiders which never have more 

 than four pairs of legs, and with insects which always 

 possess only three pairs of legs, we find this law, for 

 which a great number of examples could be adduced, con- 

 firmed. The numerical diminution of pairs of legs is a 

 progress in the organization of articulated animals. In 

 like manner the numerical diminution of corresponding 

 vertebral joints in the trunk of vertebrate animals is a 

 progress in their organization. Fishes and amphibious 

 animals with a very large number of identical vertebral 

 joints are, for this very reason, less perfect and lower than 

 birds and mammals, in which the vertebral joints, as a 



