50 BirERFECTION OF THE [Chap. X 



pigeon ; but we should have no varieties directly 

 intermediate between the fantail and pouter; none, 

 for instance, combining a tail somewhat expanded with 

 a crop somewhat enlarged, the characteristic features of 

 these two breeds. These two breeds, moreover, have 

 become so much modified, that, if we had no historical 

 or indirect evidence regarding their origin, it would not 

 have been possible to have determined, from a mere 

 comparison of their structure with that of the rock- 

 pigeon, C. li^da, whether they had descended from this 

 species or from some other allied form, such as C. oenas. 



So, with natural species, if we look to forms very 

 distinct, for instance to the horse and tapii-, we have 

 no reason to suppose that links directly intermediate 

 between them ever existed, but between each and an 

 unknown common parent. The common parent will 

 have had in its whole organisation much general re- 

 semblance 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 inter- 

 mediate links. 



It is just possible by the 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 

 remained for a very long period unaltered, whilst its 

 descendants had undergone avast amount of change; 



