RECAPITULATION AND CONCLUSION 421 



have varied under domestication. And if there has been any vari- 

 ability under nature, it would be an unaccountable fact if natural 

 selection had not come into play. It has often been asserted, but 

 the assertion is incapable of proof, that the amount of variation 

 under nature is a strictly limited quantity. Man, though acting 

 on external characters alone and often capriciously, can produce 

 within a short period a great result by adding up mere individual 

 differences in his domestic productions; and every one admits that 

 species present individual differences. But, beside such differences, 

 all naturalists admit that natural varieties exist, which are con- 

 sidered sufficiently distinct to be worthy of record in systematic 

 works. No one has drawn any clear distinction between individual 

 differences and slight varieties; or between more plainly marked 

 varieties and sub-species and species. On separate continents, and 

 on different parts of the same continent, when divided by barriers 

 of any kind, and on out-lying islands, what a multitude of forms 

 exist, which some experienced naturalists rank as varieties, others 

 as geographical races or sub-species, and others as distinct though 

 closely allied species! 



If, then, animals and plants do vary, let it be ever so slightly 

 or slowly, why should not variations or individual differences, 

 which are in any way beneficial, be preserved and accumulated 

 through natural selection, or the survival of the fittest? If man 

 can by patience select variations useful to him, why, under chang- 

 ing and complex conditions of life, should not variations useful to 

 nature's living products often arise, and be preserved or selected? 

 What limit can be put to this power, acting during long ages and 

 rigidly scrutinizing the whole constitution, structure, and habits of 

 each creature, favoring the good and rejecting the bad? I can see 

 no limit to this power, in slowly and beautifully adapting each 

 form to the most complex relations of life. The theory of natural 

 selection, even if we look no further than this, seems to be in the 

 highest degree probable. I have already recapitulated, as fairly as 

 I could, the opposed difficulties and objections: now let us turn to 

 the special facts and arguments in favor of the theory. 



On the view that species are only strongly marked and perma- 

 nent varieties, and that each species first existed as a variety, we 

 can see why it is that no line of demarcation can be drawn be- 

 tween species, commonly supposed to have been produced by 

 special acts of creation, and varieties which are acknowledged to 

 have been produced by secondary laws. On this same view we can 

 understand how it is that in a region where many species of a 



