We are as yet profoundly ignorant of
the many occasional means of transport. With respect to distinct species
of the same genus, inhabiting distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration will
have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in
some degree lessened.
As according to the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our existing varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With respect
to existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover DIRECTLY connecting links between
them, but only between each and some extinct and supplanted form. Even on
a wide area, which has during a long period remained continuous, and of
which the climatic and other conditions of life change insensibly in
proceeding from a district occupied by one species into another district
occupied by a closely allied species, we have no just right to expect often
to find intermediate varieties in the intermediate zones. For we have
reason to believe that only a few species of a genus ever undergo change;
the other species becoming utterly extinct and leaving no modified progeny.
Pages:
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748