lling 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 are 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-form 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 most continents have 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 ge