NATURAL SELECTION 159 



in the process of formation, or are, as I have called them, 

 incipient species. How, then, does the lesser difference 

 between varieties become augmented into the greater dif- 

 ference between species? That this does habitually hap- 

 pen, we must infer from most of the innumerable spe- 

 cies throughout nature presenting well-marked differences, 

 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 de- 

 gree; 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 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, 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 shorter 

 beak; another fancier is struck by a pigeon havmg 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 his- 



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