412 RECAPITULATION. [Chap. XV. 



physical changes, we might have expected to find that organic 

 beings have varied under nature, in the same way as they have 

 varied under domestication. And if there has been any variability 

 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, besides such differences, all 

 naturalists admit that natural varieties exist, which are considered 

 sufficiently distinct to be worthy of record in systematic works. 

 No one has drawn any clear distinction between individual differ- 

 ences 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 outlying islands, what a multitude of forms exist, 

 which some experienced naturalists rank as varieties, others ai 

 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 he 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 permanent 

 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 acts 

 oi creation, and varieties which are acknowledged to have been 



