These two breeds, moreover, have become
so much modified, that if we had no historical or indirect evidence
regarding their origin, it would not have been possible to have
determined from a mere comparison of their structure with that of the
rock-pigeon, whether they had descended from this species or from some
other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links ever existed directly intermediate between them, but between
each and an unknown common parent. The common parent will have had in
its whole organisation much general resemblance to the tapir and to
the horse; but in some points of structure may have differed
considerably from both, even perhaps more than they differ from each
other. Hence in all such cases, we should be unable to recognise the
parent-form of any two or more species, even if we closely compared
the structure of the parent with that of its modified descendants,
unless at the same time we had a nearly perfect chain of the
intermediate links.
It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case DIRECT intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount
of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare
event; for in all cases the new and improved forms of life will tend
to supplant the old and unimproved forms.
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