276 THE PHILOSOPHY OF BIOLOGY 



wasps that we know, just as a Copepod as large as a 

 shark would be a more formidable animal than the 

 fish. It seems possible that the reason for the smaller 

 size of the Vertebrate is to be found in the nature of the 

 skeleton. Powerful muscles would require a very- 

 strong and thick carapace, and this would attain a mass 

 in a very large insect or crustacean which would require 

 too much energy for its rapid transport. A rigid 

 exoskeleton like that of an Arthropod also means that 

 growth must take place by a process of ecdysis, that is, 

 the animal grows only during the periods when it casts 

 its shell ; and the necessity of this process of ecdysis 

 must be a formidable disadvantage in the case of a 

 very large animal, if indeed it would be possible at all. 

 Thus the Arthropod developing an exoskeleton must 

 remain small, and this smallness, fortunately for the 

 Vertebrate, has made it the less formidable animal. 

 It was an accident of evolution that the Arthropods 

 developed an exoskeleton instead of an endoskeleton. 



Undoubtedly the internal skeleton of the Verte- 

 brates, with its light, hollow, cancellated bones, was 

 mechanically the best means for the attachment of 

 muscles. It made possible a greater degree of freedom 

 of movement of the parts of the body, greater variety 

 and plasticity of action, and it removed, to some extent, 

 the limit of size and the embarrassing discontinuity of 

 growth by ecdysis, with all the dangers that this 

 involves. Above all, it led to the increased complexity 

 of the central nervous system, since this became bound 

 up with the increasing variety of bodily movement. 



In the evolution of the dominant groups of organ- 

 isms we see, then, the development of several tendencies. 

 First, that tendency which seems to offer the greatest 

 contrast to the universal tendency displayed in in- 

 organic processes, the dissipation of energy. The 



