XI PHENOMENA OF ORGANIC NATURE 441 



and take part in the struggle with the forty-nine 

 hundred, or forty-nine thousand, with which they 

 might be exposed. Thus, by degrees, this variety 

 with some slight organic change or modification, 

 must spread itself over the whole surface of the 

 habitable globe, and extirpate or replace the other 

 kinds. That is what is meant by NATURAL 

 SELECTION ; that is the kind of argument by which 

 it is perfectly demonstrable that the conditions of 

 existence may play exactly the same part for 

 natural varieties as man does for domesticated 

 varieties. No one doubts at all that particular 

 circumstances may be more favourable for one 

 plant and less so for another, and the moment you 

 admit that, you admit the selective power of 

 nature. Now, although I have been putting a 

 hypothetical case, you must not suppose that I 

 have been reasoning hypothetical! y. There are 

 plenty of direct experiments which bear out what 

 we may call the theory of natural selection ; there 

 is extremely good authority for the statement that 

 if you take the seed of mixed varieties of wheat 

 and sow it, collecting the seed next year and sow- 

 ing it again, at length you will find that out of all 

 your varieties only two or three have lived, or per- 

 haps even only one. There were one or two 

 varieties which were best fitted to get on, and they 

 have killed out the other kinds in just the same 

 way and with just the same certainty as if you had 

 taken the trouble to remove them. As I have 



