NATURAL SELECTION 1505 



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 difference between species? That this 

 does habitually happen, we must infer from most of 

 the innumerable species throughout nature present- 

 ing well-marked differences, whereas varieties, the 

 supposed prototypes and parents of future well- 

 marked species, present slight and ill-defined dif- 

 ferences. 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 ac- 

 count for so habitual and large a degree of differ- 

 ence 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 genera- 

 tions. 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 



