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 hypothetically. 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 asif you had 
taken the trouble to remove them. As I have 
