THEORY OF NATURAL SELECTION 257 



species, to which it would apparently have been advantage- 

 ous? But it is unreasonable to expect a precise answer to 

 such questions, considering our ignorance of the past history 

 of each species, and of the conditions which at the present 

 day determine its numbers and range. In most cases only 

 general reasons, but in some few cases special reasons, can 

 be assigned. Thus to adapt a species to new habits of life, 

 many co-ordinated modifications are almost indispensable, 

 and it may often have happened that the requisite parts did 

 not vary in the right manner or to the right degree. Many 

 species must have been prevented from increasing in numbers 

 through destructive agencies, 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 structures, they could not have been acquired 

 through natural selection. In many cases complex and long- 

 enduring conditions, often of a peculiar nature, are necessary 

 for the development of a structure ; and the requisite con- 

 ditions may seldom have concurred. The belief that any 

 given structure, which we think, often erroneously, would 

 have been beneficial to a species, would have been gained 

 under all circumstances through natural selection, is opposed 

 to what 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 insufficient" 

 to account for the phenomena which I explain by its agency. 

 His chief arguments have now been considered, and the 

 others will hereafter be considered. They seem to me to par- 

 take little of the character of demonstration, and to have 

 little weight in comparison with those in favour of the power 

 of natural selection, aided by the other agencies often speci- 

 fied. I am bound to add, that some of the facts and argu- 

 ments here used by me, have been advanced for the same 

 purpose in an able article lately published in the 'Mcdico- 

 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 



I— HC XI 



