132 THE GENESIS OF SPECIES. [CHAP. 



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

 and the existence of such is just what is contended for in 

 this chapter. As to the differences between domestic races 

 being greater than those between species, or even genera, 

 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 infinitesimal changes, so that 

 we ought to meet with sufficient changes to warrant the 

 belief that a hornbill could be produced from 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. 

 In a bird which has been kept and studied like the pigeon, 

 it is difficult to believe that any remarkable spontaneous 

 variations would pass unnoticed by breeders, or that they 

 would fail to be attended to and developed by some one 

 fancier or otber. On the hypothesis of indefinite varia- 

 bility, it is then hard to say why pigeons with bills like 

 toucans, or with certain feathers lengthened like those of 

 trogans, or those of birds of paradise, have never been pro- 

 duced. This, however, is a question which may be settled 

 by experiment. Let a pigeon be bred with a bill like a 

 toucan's, and with the two middle tail-feathers lengthened 

 like those of the king-bird of paradise, or even let indi- 

 viduals be produced which exhibit any marked tendency 

 of the kind, and indefinite variability shall be at once con- 

 ceded. 



As yet, all the changes which have taken place in pigeons 

 are of a few definite kinds only, such as may be well con- 

 ceived to be compatible with a species possessed of a cer- 

 tain inherent capacity for considerable yet definite varia- 

 tion, a capacity for the ready production of certain degrees 

 of abnormality, which then cannot be further increased. 



Mr. Darwin himself has already acquiesced in the prop- 



