Chap. IX. GEOLOGICAL RECORD. 281 



determined from a mere comparison of their structure 

 with that of the rock-pigeon, whether 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 recog- 

 nise 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 in- 

 stance, 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, will render this a 

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

 forms of life will tend to supplant the old and unim- 

 proved 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 be- 

 tween the varieties of the same species at the present 



