132 THE GENESIS OF SPECIES. [CHAP. 



Here again the possible limit, compatible with a healthy 

 existence, has probably been reached. In like manner, the 

 differences in the size and form of the beak in the various 

 breeds of the domestic pigeon is greater than that between 

 the extreme forms of beak in the various genera and sub- 

 families of the whole pigeon tribe. From these facts, and 

 many others of the same nature, we may fairly infer, that 

 if rigid selection were applied to any organ, we could in a 

 comparatively short time produce a much greater amount 

 of change than that which occurs between species and 

 species in a state of nature, since the differences which we 

 do produce are often comparable with those which exist 

 between distinct genera or distinct families." 



But in a domestic bird like the fantail where Natural 

 Selection does not come into play, the tail-feathers could 

 hardly be limited by " utility for flight," yet two more tail- 

 feathers could certainly exist in a fancy breed if "utility 

 for flight" were the only obstacle. It seems probable that 

 the real barrier is an internal one in the nature of the 

 organism, and the existence of such is just what is con- 

 tended for in this chapter. As to the differences between 

 domestic races being greater than those between species or 

 even genera of wild animals, that is not enough for the 

 argument. For upon the theory of " Natural Selection " 

 all birds have a common origin, from which they diverged 

 by insignificant modifications, so that we ought to meet 

 with changes sufficient to warrant the belief that a hornbill 



O 



could be produced from a creature as different from it as 

 a humming-bird, proportionate time being allowed. 



But not only does it appear that there are barriers which 

 oppose change in certain directions, but that there are posi- 

 tive tendencies to development along certain special lines. 



