CHAP. VII.] THEORY OF NATURAL SELECTION. 187 



quite the same purpose, became attached to the free ends of the 

 caudicles; all these gradations being of manifest benefit to the 

 plants in question. With respect to climbing plants, I need not 

 repeat what has been so lately said. 



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, con- 

 sidering 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, mary 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 selec- 

 tion 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.' 



At the present day almost all naturalists admit evolution under 

 some torm. 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 evolutionists; but there is no need, as it 

 seems to me, to invoke any internal force beyond the tendency to 

 ordinary variability, which through the aid of selection by man 



