94 CONDITIONS AFFECTING THE 



of the process is the same as if some person had nurtured 

 the one and destroyed the other seeds. It does not matter 

 how the variation is produced, so long as it is once allowed 

 to occur. The variation in the plant once fairly started 

 tends to become hereditary and reproduce itself ; the seeds 

 would spread themselves in the same way 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 sowing it again, at length you will find that 

 out of all your varieties only two or three have lived, or 

 perhaps 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 already said, the operation of nature is 

 exactly the same as the artificial operation of man. 



But if this be true of that simple case, which I put 

 before you, where there is nothing but the rivalry of one 

 member of a species with others, what must be the opera- 

 tion of selective conditions, when you recollect as a matter 

 of fact, that for every species of animal or plant there are 

 fifty or a hundred species which might all, more or less, be 

 comprehended in the same climate, food, and station ; 

 that every plant has multitudinous animals which prey 

 upon it, and which are its direct opponents ; and that 

 these have other animals preying upon them, that every 

 ulant has its indirect helpers in the birds that scatter 



