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 tlie few suggestive facts here 

 referred to (and many others of like import are to be found in 

 Mr. Darwin'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 b}^ 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 iii 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 



