DARWIN : UNITY OF TYPE 235 



"than that the hand of a man, formed for grasping, that of a 

 mole for digging, the leg of a horse, the paddle of the por- 

 poise, and the wing of the bat, should all be constructed on 

 the same pattern, and should include similar bones, in the 

 same relative positions? Geoffrey St Hilaire has strongly 

 insisted on the high importance of relative position or con- 

 nection in homologous parts ; they may differ to almost any 

 extent in form and size, and yet remain connected together 

 in the same invariable order" (p. 434). 



The unity of plan cannot be explained on teleological 

 grounds, as Owen has admitted in his Nature of Limbs ', nor 

 is it explicable on the hypothesis of special creation (p. 435). 

 It can be understood only on the theory that animals are 

 descended from one another and retain for innumerable 

 generations the essential organisation of their ancestors. 

 " The explanation is to a large extent simple on the theory 

 of the selection of successive slight modifications each 

 modification being profitable in some way to the modified 

 form, but often affecting by correlation other parts of the 

 organisation. In changes of this nature, there will be little 

 or no tendency to alter the original pattern or to transpose 

 the parts. ... If we suppose that the ancient progenitor, the 

 archetype as it may be called, of all animals, had its limbs 

 constructed on the existing general pattern, for whatever 

 purpose they served, we can at once perceive the plain 

 significance of the homologous construction of the limbs 



o o 



throughout the whole class" (p. 435). 



We may note three important points in this passage 

 first, the identification of the archetype with the common 

 progenitor ; second, the view that progressive evolution is 

 essentially adaptive, and dominated by natural selection ; 

 and third, the petitio principii involved in the assumption 

 that adaptive modification brings inevitably in its train 

 the necessary correlative changes. 



In his section on morphology Darwin shows clearly the 

 influence of Owen, and through him of the transcendental 

 anatomists. He refers to the transcendental idea of " meta- 

 morphosis," as exemplified in the vertebral theory of the 

 skull and the theory of the plant appendage, and shows how, 

 on the hypothesis of descent with modification, " meta- 



