, 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 {281} 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