72 THE CLASS OF INSECTS. 



zone, whose summits nearly reach the snow-line, we find a 

 few insects which are the same or very similar to those of the 

 polar regions ; such an assemblage is called an Alpine fauna. 



The insect-fauna of each great continent may be divided into 

 an Arctic, or polar, a Temperate, and a Tropical fauna, and an 

 Alpine fauna if there are mountains in the warm latitudes which 

 reach near the snow-line. Mountain barriers, inland seas, des- 

 erts, and peculiarities in the flora (or collection of plants 

 peculiar to a certain district), are boundaries of secondary 

 importance in limiting the distribution of species. 



On the other hand insects are diffused by winds, rivers, 

 oceanic currents, and the agency of man. By the latter im- 

 portant means certain insects become cosmopolitan. Certain 

 injurious insects become suddenly abundant in newly cultivated 

 tracts. The balance of nature seems to be disturbed, and 

 insects multiplying rapidly in newly settled portions of the 

 country, become terrible pests. In the course of time, how- 

 ever, they seem to decrease in numbers and moderate their 

 attacks. 



Insect-faunae are not limited by arbitrary boundaries, but 

 fade into each other by insensible gradations corresponding in 

 a general way to the changes of the temperature of different 

 portions of the district they inhabit. 



The subject of the geographical distribution of insects, of 

 which we have as yet but given the rudiments, may be studied 

 to great advantage in North America. The Arctic insect-fauna 

 comprises Greenland, the arctic American Archipelago, and the 

 northern shores of the continent beyond the limit of trees. A 

 large proportion of the insects found in this region occur in 

 arctic Europe and arctic Asia, and are hence called circum- 

 polar, while other species are indigenous to each country. 

 Again, the arctic fauna of Labrador and Hudson's Bay differs 

 from that of the arctic portions of the region about Behring's 

 Straits, certain species characterizing one side of the continent 

 being replaced by representative species which inhabit the 

 opposite side. 



The Alpine fauna of the White Mountains consists, besides 

 a very few peculiar to them, of circumpolar species, which are 

 now only found in Labrador and Greenland, and which are 



