312 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



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 modifi- 

 cations 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 advan- 

 tageous 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 conditions may seldom have concurred. 

 The belief that any given structure, which we thinly 

 often erroneouslv, 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 partake 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 specified. I am bound to add, that 

 some of the facts and arguments here used by me, have 

 been advanced for the same purpose in an able article 

 lately published in the ' Medico-Chirurgical Review.' 



