CHAP. IV.] DIVERGENCE OF CHARACTER. 81 



whereas varieties, the supposed prototypes and parents of future 

 well-marked species, present slight and ill-defined differences. 

 Mere 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 some- 

 thing analogous. It will be admitted that the production of races 

 so different as short-horn and Hereford cattle, race and cart 

 horses, the several breeds of pigeons, &c., 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 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 dis- 

 trict required swifter horses, whilst 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 in- 

 ferior 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 principle apply in 

 nature 1 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 

 become 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. 



