ffal, or loads the viscera with fat, or gives early
maturity, &c.,--the chances are strong that he will at the same time weaken
the constitution. On the other hand, when an animal has to struggle
throughout its life with many competitors and enemies, under circumstances
inconceivably complex and liable to change, modifications of the most
varied nature--in the internal organs as well as in external characters, in
the {246} functions and mutual relations of parts--will be rigorously
tested, preserved, or rejected. Natural selection often checks man's
comparatively feeble and capricious attempts at improvement; and if this
were not so, the result of his work, and of nature's work, would be even
still more different. Nevertheless, we must not overrate the amount of
difference between natural species and domestic races; the most experienced
naturalists have often disputed whether the latter are descended from one
or from several aboriginal stocks, and this clearly shows that there is no
palpable difference between species and races.

Domestic races propagate their kind far more truly, and endure for much
longer periods, than most naturalists are willing to admit. Breeders feel
no doubt on this head; ask a man who has long reared Shorthorn or Hereford
cattle, Leicester or Southdown sheep, Spanish or Game poultry, tumbler or
carrier-pigeons, whether these races may not have been derived from common
progenitors, and he will probably laugh you to scorn. The breeder admits
that he may hope to produce sheep with finer or longer wool and with better
carcases, or handsomer fowls, or carrier-pigeons with beaks just
perceptibly longer to the practised eye, and thus be successful at an
exhibition. Thus far he will go, but no farther. He does not reflect on
what follows from adding up during a long course of time many, slight,
successive modifications; nor does he reflect on the former existence of
numerous varieties, connecting the links in each divergent line of descent.
He concludes, as was shown in the earlier chapters, that all the chief
breeds to which he has long attended are aboriginal productions. The
systematic naturalist, on the other hand, who generally knows nothing of
the art of breeding, who does not pretend to know how and when the several
domestic races were formed, who cannot have seen the intermediate
gradations, for they do not now exist, nevertheless feels no doubt that
these races are sprung from a single source. Bu