34^ THE EVOLUTION OF MAN. 



various functions are distributed among the various kinds oi 

 carriages — functions which each separate carriage can- 

 not discharge simultaneously — so in the articulated animal 

 body the division of labour among the metamera of the 

 trunk must be regarded as a material advance. 



The best explanation of the nature of metameric 

 formation is afforded by the articulated Worms, especially 

 the Tape-worms and the Ringed- worms (Annelida). In 

 these the members, or metamera, composing the ringed body, 

 are all of the same structure and of the same form-value. 

 The first member, the head, alone seems to be differently 

 formed and more or less differentiated. In many Tape- 

 worms the various members are so independent, that many 

 zoologists regard each separate metameron as an individual 

 animal, and the whole chain of members as a colony o A 

 animals. In a certain sense this is quite correct, in so far 

 as each separate metameron is an individual of a lower 

 order, while the chain, composed of many metamera, is an 

 individual of a higher order. But in proportion as the 

 separate members relinquish their independence ; in pro- 

 portion as they become differentiated in consequence ol 

 division of labour, and become dependent on each other 

 and on the whole body, and in proportion as the latter 

 becomes centralized, the more perfect does the entire 

 unitary organism become. In most Articulated .Animals 

 (Arthropoda), and in all Vertebrates, centralization has so 

 far progressed that the individual metamera are no longer 

 of any importance in themselves alone, and are to be con- 

 sidered merely as the necessary component parts of the 

 entire chain. 



When we investigate the origin of the metameric chain 



