CHAPTEE 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 our 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 



