THE MEANING OF EVOLUTION 275 



development of powerful pectoral muscles, did in- 

 definite possibilities of flight reside ; and only in the 

 fish, with the concomitant evolution of gills, the re- 

 duction of a minimum of the mass of the alimentary 

 canal and its glands, and the conversion of most of 

 the muscles of the body into organs actuating the tail 

 fin, was the completeness of adaptation to aquatic 

 life realised. Mobility, a bodily structure capable of 

 indefinitely varied movements, and a nervous system 

 by the aid of which any part of the body might become 

 linked to any other part — these were the structural 

 adaptations that have been successful alike in 

 Arthropod and Vertebrate. 



There were apparently two main types of structure 

 by means of which this mobility and elasticity could 

 be attained, the Arthropod type and the Vertebrate 

 type. There seems little to choose between them if 

 we had to select one of them in order to obtain a highly 

 mobile organic mechanism. Arthropod and Vertebrate 

 seem to be equally complex if we take account of 

 difference in size and the additional bodily mechanism 

 that great size must involve. Certainly the muscula- 

 ture of the Vertebrate is more complex than in the 

 Arthropod. But greater weight must require larger 

 and more powerful muscles if the same degree of 

 mobility relative to the size of the animal is to be 

 attained, and this more complex musculature must 

 carry with it a more complex brain. It must also be 

 concomitant with a more massive skeleton, for rigid 

 supports for the muscles must be present in the mechan- 

 ism. Why are there no great insects or crustaceans ? 

 Mr Wells has suggested in one of his novels the for- 

 midability of a wasp two feet long ! Such a creature 

 would indeed be more dreadful than any predatory 

 bird that we know if its activity were also that of the 



