INSECT EVOLUTION CHETVERIKOV. 449 



is left for the musculature, which is, of course, especially unsuited 

 for a massive, bony skeleton. 



We thus see that in both instances there is a decided and tremen- 

 dous advantage in the insect skeleton. It is only, thanks to these ad- 

 vantages of their external skeleton, that insects weit able to develop 

 those small, slender, harmonious, and exquisite forms, the perfection 

 of which we so often admire and whither the vertebrates with their 

 heavy, clumsy endoskeleton were, of course, unable to follow them. 

 And if we add the fact that the exoskeleton represents besides an end- 

 less field for the development of purely external characters, then the 

 great variation of contemporary insects should no longer surprise us. 

 But, of course, these forms could develop only gradually, by means 

 of a long and slow evolution, and this is why we do not meet them 

 among the early primitive, cumbersome, and clumsy Carboniferous 

 forms. 



We have reached the end of our thesis. If we now return to the 

 question with which we began as to the fundamental cause of the 

 opposite direction of the paths of evolution of vertebrates and in- 

 sects we will find, it seems to me, but one answer: The cause is, the 

 presence in insects of an outer chitinous skeleton, owing to which they 

 were in a position, by continuously diminishing the dimension of their 

 body, to conquer for themselves an entirely independent place among 

 the other terrestrial animals, and not only to conquer it but to in- 

 crease in an endless variation of forms and thereby acquire a tremen- 

 dous importance in the general economy of nature. Thus their in- 

 significance became their power. 



