NATURAL SELECTION 87 



chance, as we may call it, might cause one variety to differ in some 

 character from its parents, and the offspring of this variety again 

 to differ from its parent in the very same character and in a greater 

 degree; but this alone would never account for so habitual and 

 large a degree of difference as that between the species of the same 

 genus. 



As has always been my practice, I have sought light on this head 

 from our domestic productions. We shall here find something an- 

 alogous. It will be admitted that the production of races so dif- 

 ferent as short-horn and Hereford cattle, race and cart horses, the 

 several b r eeds of pigeons, etc., could never have been effected by 

 the mere chance accumulation of similar variations during many 

 successive generations. In practice, a fancier is, for instance, struck 

 by a pigeon having a slightly snorter beak; another fancier is 

 struck by a pigeon having a rather longer beak; and on the ac- 

 knowledged 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) choos- 

 ing 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. Ulti- 

 mately, after the lapse of centuries, these sub-breeds would be- 

 come converted into two well-established and distinct breeds. As 

 the differences became greater, the inferior animals with inter- 

 mediate characters, being neither very swift nor very strong, would 

 not have been used for breeding, and will thus have tended to dis- 

 appear. 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 principle 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 diversified the descendants from any one species be- 

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

 be better enabled to seize on many and widely diversified places 

 in the polity of nature, and so be enabled to increase in numbers. 



