DKAGONS OF THE DEEP. 207 



dactyles and the Dinosaurs, as well as the creatures; 

 described in the present chapter, are all originally 

 tooth-bearing forms, but each of these groups 

 presents at least a considerable number of tooth- 

 less types. At the same time, we would make 

 it perfectly clear that it is not to be supposed 

 that extinction was in any case actually due to 

 the loss of the teeth ; we only wish to indicate 

 how easily extinction may be brought about 

 among highly specialised types from their in- 

 ability to vary further to meet new demands. 

 The fact that the tooth-bearing forms suffered 

 extinction equally with the toothless types, shows, 

 the fallacy of such a supposition. 



Some extremely valuable suggestions on the 

 causes of extinction, recently made by Mr C. B. 

 Crampton and Dr C. W. Andrews, may fittingly 

 be reviewed here. According to the first- 

 mentioned author, the possible variations of an 

 organism become less and less as specialisation 

 advances ; on this account any change in the 

 environment threatens the life of the species,, 

 since only a very few individuals will have 

 chanced to vary in a direction favourable to 

 the changed circumstances. As a consequence 

 of this process of weeding out, the stock becomes, 

 more and more highly specialised, and therefore 

 more and more physiologically similar, "until 

 at length the same results as arise from close 

 inter-breeding weakening of the stock, and 

 finally extinction. Yet another possible factor 

 in the process of extermination is that suggested 

 by Dr Andrews. Discussing the evolution of the 

 elephants he pointed out, another possible cause 



