262 ON THE ORIGIN OF SPECIES 



they had descended from this species or from some 

 other allied species, such as C. oenas. 



So with natural species, if we look to forms very 

 distinct, for instance to the horse and tapir, we have 

 no reason to suppose that links ever existed directly 

 intermediate between them, but between each and an 

 unknown common parent. The common parent will 

 have had in its whole organisation much general resem- 

 blance to the tapir and to the horse ; but in some 

 points of structure may have differed considerably from 

 both, even perhaps more than they differ from each 

 other. Hence in all such cases, we should be unable 

 to recognise the parent -form of any two or more 

 species, even if we closely compared the structure of 

 the parent with that of its modified descendants, unless 

 at the same time we had a nearly perfect chain of the 

 intermediate links. 



It is just possible by my theory, that one of two 

 living forms might have descended from the other ; for 

 instance, a horse from a tapir ; and in this case direct 

 intermediate links will have existed between them. 

 But such a case would imply that one form had re- 

 mained for a very long period unaltered, whilst its 

 descendants had undergone a vast amount of change ; 

 and the principle of competition between organism and 

 organism, between child and parent, willrenderthisavery 

 rare event ; for in all cases the new and improved forms 

 of life tend to supplant the old and unimproved forms. 



By the theory of natural selection all living species 

 have been connected with the parent-species of each 

 genus, by differences not greater than we see between 

 the varieties of the same species at the present day ; 

 and these parent-species, now generally extinct, have 

 in their turn been similarly connected with more 

 ancient species ; and so on backwards, always con- 

 verging to the common ancestor of each great class. 

 So that the number of intermediate and transitional 

 links, between all living and extinct species, must have 

 been inconceivably great But assuredly, if this theory 

 be true, such have lived upon this earth. 



