NATURAL SELECTION. 109 



for instance, struck by a pigeon having a slightly shorter 

 beak ; another fancier is struck by a pigeon having a 

 rather longer beak ; and, on the acknowledged principle 

 that "fanciers do not and will not admire a medium 

 standard, but like extremes," they both go on (as has 

 actually occurred with the sub-breeds of the tumbler- 

 pigeon) choosing and breeding from birds with longer 

 and longer beaks, or with shorter and shorter beaks. 

 Again, we may suppose that, at an early period of history, 

 the men of one nation or district required swifter horses, 

 while those of another required stronger and bulkier 

 horses. The early differences would be very slight ; but, 

 in the course of time, from the continued selection of 

 swifter horses in the one case, and of stronger ones in the 

 other, the differences would become greater, and would 

 be noted as forming two sub-breeds. Ultimately, after 

 the lapse of centuries, these sub-breeds would become 

 converted into two well-established and distinct breeds. 

 As the differences became greater, the inferior animals 

 with intermediate characters, being neither very swift 

 nor very strong, would not have been used for breeding, 

 and will thus have tended to disappear. Here, then, we 

 see in man's productions the action of what may be called 

 the principle of divergence, causing differences, at first 

 barely appreciable, steadily to increase, and the breeds to 

 diverge in character, both from each other and from their 

 common parent. 



But how, it may be asked, can any analogous prin- 

 ciple apply in nature ? I believe it can and does apply 

 most efficiently (though it was a long time before I saw 

 how), from the simple circumstance that the more diver- 

 sified the descendants from any one species become in 

 structure, constitution, and habits, by so much will they 

 be better enabled to seize on many and widely diversified 



