484 CONCLUSIONS 



in any other environment but their own. Once committed, 

 they are committed for always, for in its broad lines evolution 

 is irreversible. 



Primitive animals, on the other hand, are not committed to 

 any particularly restricted mode of life ; they do not have any 

 delicate adaptations with the structural modifications which 

 they involve, and they are, in a word, generalised. 



It is from generalised ancestors that the main groups of 

 animals have evolved, and as these groups radiated out they 

 became specialised in their various ways. Specialisation and 

 evolutionary capacity are roughly inversely proportional. 



The significance of primitiveness and specialisation is thus 

 related to evolution. Amphioxus is primitive because it 

 possesses many characters which the early ancestral Chordates 

 must have had. But its specialised characters show that it was 

 not itself that ancestor. Amphioxus is with regard to the higher 

 Chordates not a " father " but an " uncle." 



It is worth noticing that the primitive arrangement of several 

 structures was segmental, and that as evolution proceeded this 

 simple scheme was departed from. So the gonads of Amphi- 

 oxus, myotomes of Amphioxus, kidney tubules of Myxine, 

 ribs of Cotylosaurs and respiratory centres of Raia show that 

 " a pair of each in each segment " was the primitive outfit, 

 on which evolution has worked. 



When man is considered in relation to his ancestors, a 

 significant fact emerges. Man is not adapted to any restricted 

 mode of life at all ; instead he is fitted for almost all sorts of 

 habits and circumstances ; he is generalised not specialised, 

 and that is one of the secrets of his evolutionary success. His 

 ancestors must have been among the most primitive and 

 generalised of the mammals ; they did not live on the capital of 

 their evolutionary capacities and spend it in exchange for 

 delicate adaptations, which, while perhaps allowing of " easier 

 living," would have resulted in side-tracking the race into a rut 

 or backwater of life. 



Lastly, mention may be made of the material which the 

 vertebrates supply for a consideration of what is often called 

 the Law of Recapitulation. It is not astonishing that a group 



