mediate and linear progenitors of those now existing. Recent as
the yet discovered fossil mammifers of S. America are, who will pretend
to say that very many intermediate forms may not have existed? Moreover,
we shall see in the ensuing chapter that the very existence of genera
and species can be explained only by a few species of each epoch leaving
modified successors or new species to a future period; and the more
distant that future period, the fewer will be the _linear_ heirs of the
former epoch. As by our theory, all mammifers must have descended from
the same parent stock, so is it necessary that each land now possessing
terrestrial mammifers shall at some time have been so far united to
other land as to permit the passage of mammifers{417}; and it accords
with this necessity, that in looking far back into the earth's history
we find, first changes in the geographical distribution, and secondly a
period when the mammiferous forms most distinctive of two of the present
main divisions of the world were living together{418}.

{416} _Origin_, Ed. i. p. 341, vi. p. 487.

{417} _Origin_, Ed. i. p. 396, vi. p. 549.

{418} _Origin_, Ed. i. p. 340, vi. p. 486.

I think then I am justified in asserting that most of the above
enumerated and often trivial points in the geographical distribution of
past and present organisms (which points must be viewed by the
creationists as so many ultimate facts) follow as a simple consequence
of specific forms being mutable and of their being adapted by natural
selection to diverse ends, conjoined with their powers of dispersal, and
the geologico-geographical changes now in slow progress and which
undoubtedly have taken place. This large class of facts being thus
explained, far more than counterbalances many separate difficulties and
apparent objections in convincing my mind of the truth of this theory of
common descent.

_Improbability of finding fossil forms intermediate between existing
species._

There is one observation of considerable importance that may be here
introduced, with regard to the improbability of the chief transitional
forms between any two species being found fossil. With respect to the
finer shades of transition, I have before remarked that no one has any
cause to expect to trace them in a fossil state, without he be bold
enough to imagine that geologists at a future epoch will be able to
trace from fossil bones the gradations between the Short-H