314 Darwin, and after Darwin. 



resemble natural species in all other respects, never- 

 theless present one conspicuous and highly important 

 point of difference : they rarely, if ever, present the 

 physiological character of mutual infertility, which is 

 a character of extremely general occurrence in the 

 case of natural species, even when these are most 

 nearly allied. 



I will deal with these two objections in the next 

 chapter, where I shall be concerned with the meeting 

 of all the objections which have ever been urged 

 against the theory of natural selection. Meanwhile I 

 am engaged only in presenting the general arguments 

 which support the theory, and therefore mention these 

 objections to one of them merely en passant. And I 

 do so in order to pledge myself effectually to dispose 

 of them later on, so that for the purposes of my present 

 argument both these objections may be provisionally 

 regarded as non-existent ; which means, in other 

 words, that we may provisionally regard the analogy 

 between artificial selection and natural selection as 

 everywhere logically intact. 



To sum up, then, the results of the foregoing 

 exposition thus far, what I hold to be the three 

 principal, or most general, arguments in favour of the 

 theory of natural selection, are as follows. 



First, there is the a priori consideration that, if on 

 independent grounds we believe in the theory of 

 evolution at all, it becomes obvious that natural 

 selection must have had some part in the process. 

 For no one can deny the potent facts of heredity, 

 variability, the struggle for existence, and survival of 

 the fittest. But to admit these facts is to admit 



