OBJECTIONS TO THE THEORY 839 



It has often been asked, if natural selection "be so 

 potent why has not this or that structure been gained 

 by certain species, to which it would apparently have 

 been advantageous ? But it is unreasonable to expect a 

 precise answer to such questions, considering our igno- 

 rance of the past history of each species, and of the con- 

 ditions 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 conditions 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 insuffi- 

 cient" to account for the phenomena which I explain by 

 its agency. His chief arguments have now been consid- 



