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 contended for in this chapter. As to{118}
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 positive 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 other. On the hypothesis
of _indefinite_ variability, 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 produced. 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 individuals be produced
which exhibit any marked tendency of the kind, and indefinite variability
shall be at once conceded.

As yet all the changes which have taken place in pigeons are of a few
definite kinds only, such as may be well conceived to be compatible with a
species possessed of a certain inherent capacity for considerable yet
definite variation, 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 proposition here
maintained, inasmuch as he distinctly affirms the existence of a marked
internal barrier to change in certain cases. And if this is admitted in one
case, the _principle_ is conceded, and it immediately becomes probable that
such internal barriers exist in all, although enclosing a much larger {119}
field for variation in some cases than in others. Mr. Darwin abundantly
demons