132 THE GEXESIS OF SPECIES. [Chap. 



Here again the possible limit, compatible Avitli a hoaltliy 

 existence, has ]»robably 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 ^vhole pigeon tribe. From these facts, and 

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

 if rigid selection were ap])lied to any organ, w'e 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 Xatural 

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

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

 feathers could certainly exist in a fancy breed if " utility 

 for fliglit " were the only obstacle. It seems probaljle 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 diff'erences between 

 domestic races being greater than those between species or 

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

 argument. Fur upon tlie 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 

 could be produced from a creature as different IVbm it as 

 a humming-bird, ])roportionate 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. 



