466 NINETEENTH CENTURY. PT. IIL 



together. You should fix this difficulty firmly in your mind, 

 because it is almost the only real one we shall meet with. 

 Mr. Darwin's answer to it is, that we have only watched 

 plants and animals for such a short time, and even then not 

 with this idea in our minds, so that we are not likely to 

 have found a case to help us. It has indeed been observed 

 that animals, if left free to choose, do often pair with those 

 \\hich resemble themselves, and do in some cases show a 

 dislike to those that differ ; still this is not proved to be 

 always the case, and it must be acknowledged to be a 

 difficulty. 



Selection of Animals by Man. But now setting this 

 aside, let us see what proof there is that animals vary, and 

 that they can be picked out, so that any peculiarity may be- 

 come stronger in each succeeding generation. The best 

 instance is in pigeons. All our pigeons come from the 

 common wild rock-pigeon ; and the way in which all our 

 pouters, fan-tails, barbs, and other pigeons have been pro- 

 duced, is by merely picking out from the young ones those 

 which had either large crops, or wider tails, or longer beaks, 

 and pairing them together, so that the young birds had these 

 peculiarities still more strongly. The same thing is true of 

 our different kinds of oxen, sheep, horses, and fowls ; so we 

 see clearly that different varieties can be produced by choos- 

 ing out particular animals. Man does this quickly, because 

 he only attends to one peculiarity, which inte ests him ; but 

 nature does it very slowly, because no animal can live unless 

 every part of it is fitted for its life better than in those which 

 are killed off. 



Selection by Natural Causes. Now Mr. Wallace has 

 calculated that one pair of birds having four young ones a 

 year, would, if all their children, grandchildren, and great- 

 grandchildren, lived and were equally prolific, produce about 



