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. 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. De Candolle has observed,
a common alpine species disappears. The same fact has been noticed by
Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the
all-important elements of distribution, these facts ought to cause
surprise, as climate and height or depth graduate away insensibly. But
when we bear in mind that almost every species, even in its metropolis,
would increase immensely in numbers, were it not for other competing
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