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 
