NUMMARY OF ' PHILOSOPHIE ZOOLOGTQUE: 285 



pigeon 0. livia, whether they had descended from this 

 species, or from some other allied form, as G. oenas. 



" So with natural species, if we look to forms Tery 

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

 have no reason to suppose that links directly interme- 

 diate between them ever existed, but between each and 

 an unknown common parent. The common parent will 

 have had in its whole organization much general re- 

 semblance to the tapir and the horse ; but in some 

 points of structure it 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 recognize 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. 



" By the theory of natural selection [surely this is a 

 slip for " by the theory of descent with modification "] 

 all living species have been connected with the parent 

 species of each genus, by differences not greater than we 

 see between the natural and domestic varieties of the 

 same species at the present day; and their parent 

 species, now generally extinct, have in their turn been 

 similarly connected with more ancient forms, and so on 

 backwards, always converging 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 [the theory of descent with 



