THEORY OF NATURAL iSELECTloN, 237 



which stood in no relation to certain structures, •which we 

 imagine would have been gained through natural selection 

 from appearing to us advantageous to the species. In this 

 case, as the struggle for life did not depend on such struct- 

 nres, they could not have been acquired through natural 

 selection. In many cases complex and long-enduring con- 

 ditions, often of ^a peculiar nature, are necessary for the 

 development of a structure; and the requisite conditions 

 may seldom have concurred. The belief that any given 

 structure, which we think, often erroneously, would have 

 been beneficial to a species, v^ould have been gained under 

 all circumstances through natural selection, is opposed to 

 fvhat we can understand of its manner of action. Mr. 

 Mivart does not deny that natural selection has effected 

 something; but he considers it as " demonstrably insuf- 

 ficient " to account for the phenomena which I explain by 

 its agency. His chief arguments have now been con- 

 sidered, and the others will hereafter be considered. They 

 seem to me to partake little of the character of demonstra- 

 tion, and to have little weight in comparison with those in 

 favor of the power of natural selection, aided by the other 

 agencies often specified. I am bound to add, that some of 

 the facts and arguments here used by me, have been ad- 

 vanced for the same purpose in an able article lately pub- 

 lished in the ''Medico-Chirurgical Review." 



At the present day almost all naturalists admit evolution 

 under some form. Mr. Mivart believes that species change 

 through *'an internal force or tendency," about which it 

 is not pretended that anything is known. That species 

 have a capacity for change will be admitted by all evolu- 

 tionists; but there is no need, as it seems to me, to invoke 

 anv internal force beyond the tendency to ordinary varia- 

 bility, which through the aid of selection, by man has given 

 rise to many well-adapted domestic races, and which, 

 through the aid of natural selection, would equally well 

 give rise by graduated steps to natural races or species. 

 The final result will generally have been, as already ex- 

 plained, an advance, but in some few cases a retrogression, 

 in organization. 



Mr. Mivart is further inclined to believe, and some 

 naturalists agree with him, that new species manifest 

 themselves '* with suddenness and by modifications appear- 



