force and
truth, there must be both with natural and domestic productions a limit to
change in certain directions; for instance, there must be a limit to the
fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight to be carried, and the power of
contraction in the muscular fibres. The English racehorse may have reached
this limit; but it already surpasses in fleetness its own wild progenitor,
and all other equine species.

It is not surprising, seeing the great difference between many domestic
breeds, that some few naturalists have concluded that all are descended
from distinct aboriginal stocks, more especially as the principle of
selection has been ignored, and the high antiquity of man, as a breeder of
animals, has only recently become known. Most naturalists, however, freely
admit that various extremely dissimilar breeds are descended from a single
stock, although they do not know much about the art of breeding, cannot
show the connecting links, nor say where and when the breeds arose. Yet
these same naturalists will declare, with an air of philosophical caution,
that they can never admit that one natural species has given birth to
another until they behold all the transitional steps. But fanciers have
used exactly the same language with respect to domestic breeds; thus an
author of an excellent treatise says he will never allow that carrier and
fantail pigeons are the descendants of the wild rock-pigeon, until the
transitions have "actually been observed, and can be repeated whenever man
chooses to set about the task." No doubt it is difficult to realise that
slight changes added up during long centuries can produce such results; but
he who wishes to understand the origin of domestic breeds or natural
species must overcome this difficulty.

The causes inducing and the laws governing variability have been so lately
discussed, that I need here only enumerate the leading points. As
domesticated organisms are much more {418} liable to slight deviations of
structure and to monstrosities, than species living under their natural
conditions, and as widely-ranging species vary more than those which
inhabit restricted areas, we may infer that variability mainly depends on
changed conditions of life. We must not overlook the effects of the unequal
combination of the characters derived from both parents, nor reversion to
former progenitors. Changed conditions have an especial