CHAP. XV.] RECAPITULATION. 387 



of variation under nature is a strictly limited quantity. Man, 

 though acting on external characters alone and often capri- 

 ciously, 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, 

 besides such differences, all naturalists admit that natural varie- 

 ties exist, which are considered 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 expe- 

 rienced 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 changing 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 

 scrutinising the whole constitution, structure, and habits of each 

 creature, favouring 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 farther 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 favour 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 between 

 species, commonly supposed to have been produced by special act? 

 of creation, and varieties which are acknowledged to have been 

 produced by secondary laws. On this same view we can under- 

 stand how it is that in a region where many species of a genus 

 have been produced, and where they now flourish, these same 

 species should present many varieties ; for where the manufactory 

 of species has been active, we might expect, as a general rule 

 to find it still in action; and this is the case if varieties be 

 incipient species. Moreover, the species of the larger genera, 

 which afford the greater number of varieties or incipient species, 

 retain to a certain degree the character of varieties; for they 



