46 DISTRIBUTION OF ANIMALS. [PART I, 
adapted to live, to increase, and to maintain itself under adverse 
circumstances, than the other. 
Now if we consider carefully the few suggestive facts here 
referred to (and many others of like import are to be found in 
Mr. Darwinm’s various works), we shall be led to conclude that 
the several species, genera, families, and orders, both of animals 
and vegetables which inhabit any extensive region, are bound 
together by a series of complex relations; so that the increase, 
diminution, or extermination of any one, may set in motion a 
series of actions and reactions more or less affecting a large 
portion of the whole, and requiring perhaps centuries of fluctua- 
tion before the balance is restored. The range of any species 
or group in such a region, will in many cases (perhaps in most) 
be determined, not by physical barriers, but by the competition 
of other organisms. Where barriers have existed from a remote 
epoch, they will at first have kept back certain animals from 
coming in contact with each other; but when the assemblage 
of organisms on the two sides of the barrier have, after many 
ages, come to form a balanced organic whole, the destruction of 
the barrier may lead to a very partial intermingling of the 
peculiar forms of the two regions. Each will have become 
modified in special ways adapted to the organic and physical 
conditions of the country, and will form a living barrier to the 
entrance of animals less perfectly adapted to those conditions. 
Thus while the abolition of ancient barriers will always lead 
to much intermixture of forms, much extermination and wide- 
spread alteration in some families of animals; other important 
groups will be unable materially to alter their range; or they 
may make temporary incursions into the new territory, and be 
ultimately driven back to very near their ancient limits. 
In order to make this somewhat difficult subject more intelli- 
gible, it may be well to consider the probable effects of certain 
hypothetical conditions of the earth’s surface :— 
1. If the dry land of the globe had been from the first 
continuous, and nowhere divided up by such boundaries as lofty 
mountain ranges, wide deserts, or arms of the sea, it seems 
probable that none of the larger groups (as orders, tribes, or 
