CHAPTER I. 
INTRODUCTORY. 
It is a fact within the experience of most persons, that the 
various species of animals are not uniformly dispersed over the 
surface of the country. If we have a tolerable acquaintance 
with any district, be it a parish, a county, or a larger extent of 
territory, we soon become aware that each well-marked portion 
of it has some peculiarities in its animal productions. If we 
want to find certain birds or certain insects, we have not only to 
choose the right season but to go to the right place. If we 
travel beyond out district in various directions we shall almost 
certainly meet with something new to us ; some species which 
we were accustomed to see almost daily will disappear, others 
which we have never seen before will make their appearance. 
If we go very far, so as to be able to measure our journey by 
degrees of latitude and longitude and to perceive important 
changes of climate and vegetation, the differences in the forms of 
animal life will become greater ; till at length we shall come to a 
country where almost everything will be new, all the familiar 
creatures of our own district being replaced by others more or 
less differing from them. 
If we have been observant during our several journeys, and 
have combined and compared the facts we have collected, it will 
become apparent that the change we have witnessed has been 
of two distinct kinds. In our own and immediately surround- 
ing districts, particular species appeared and disappeared because 
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