DARWIN ON THE ORIGIN OF SPECIES 129 



in the pectoral fin of the fish. But why a common plan ? 

 Of what advantage is it to an animal that its wing, 

 paddle, or hand should reproduce the general plan of a 

 fore-foot? Why should the digits of the land verte- 

 brates never exceed five ? Why should the thumb never 

 have more than two free joints ? It would be hard to find 

 a satisfactory answer to these questions in any book 

 earlier than the Origin of Species ; no student of the 

 Origin of Species finds any difficulty in answering them 

 all. The common plan has been transmitted from type 

 to type by inheritance, and its features are derived from 

 an unknown common ancestor. 



The new conception, that structures inherited from 

 remote ancestors may be incessantly modified by the 

 conditions of life and by mutual competition, is the key 

 to the chief problems of morphology. No limited collec- 

 tion of examples can substantiate so wide a proposition 

 as this. Those who have made themselves familiar with 

 old text-books of comparative anatomy will recollect how 

 dry, or else how inconclusive, was pre-evolutionary mor- 

 phology, how vague were the references to some ideal 

 archetype, or to climate, or to the ancient conditions of 

 the earth's surface ; how often exclamations of admira- 

 tion for the marvels of nature or Providence were sub- 

 stituted for clear explanations. Cuvier, it is true, was 

 both precise and reasonable ; but how little he was in 

 a position to explain! His "empirical" comparative 

 anatomy could throw no direct light upon origins or 

 transformations ; his " rational " comparative anatomy 

 was practicable only in a few easy cases. 



4. Geographical Distribution. — The facts of distribu- 

 tion were handled in the Origin of Species with great 

 originality. It was shown that they support, and 

 indeed require, some doctrine of organic evolution. 



