But I think it can be
in large part explained.
In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during
a long period. Geology would lead us to believe that almost every
continent has been broken up into islands even during the later
tertiary periods; and in such islands distinct species might have been
separately formed without the possibility of intermediate varieties
existing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have
been formed on strictly continuous areas; though I do not doubt that
the formerly broken condition of areas now continuous has played an
important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and
finally disappearing. Hence the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending mountains,
and sometimes it is quite remarkable how abruptly, as Alph.
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