intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself, forms _directly_ intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons have both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
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
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