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 ex 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 grivorzZ consideration that, if on 
independent grounds we belicve in the theory of 
evolution at all, it becomes obvious that natural 
selection mst 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 
