But it may be urged that when several closely-allied species inhabit
the same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive
intervals with closely allied or representative species, evidently
filling nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and as the one
becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species have
descended from a common parent; and during the process of
modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent
and all the transitional varieties between its past and present
states. Hence we ought not to expect at the present time to meet with
numerous transitional varieties in each region, though they must have
existed there, and may be embedded there in a fossil condition. But in
the intermediate region, having intermediate conditions of life, why
do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me.
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