DISPERSAL OF BIEDS AND INSECTS. 51 



muda Islands, which are distant from seven hundred to eight 

 hundred miles from the nearest coast, we meet with a different 

 order of things. The bird-fauna of these islands consists in all of 

 about one hundred and eighty species, including both the land and 

 aquatic forms, of which number, however, about thirty have been 

 noticed only on one occasion. Of the eighty-five species of land- 

 birds less than ten are permanent residents, the rest making their 

 way principally from the North American continent and the West 

 India islands.^' It is a singular circumstance that most of the 

 foreign invaders are strictly migrating birds, whose course of mi- 

 gration lies along the Atlantic coast, and which in their periodical 

 wanderings frequently pass at some considerable distance out to 

 sea. Entering the region of violent winds and hurricanes, they 

 are liable to be snatched from their track, and to be forcibly trans- 

 ported to some remote shore, where, of necessity, they will be 

 compelled to secure for themselves a new home, and where, through 

 frequent visitations of a like character, the original breeds estab- 

 lished will remain pure and unaltered. Such is the condition of 

 the bird-fauna of the Bermudas at the present time. None of the 

 strictly non-migratory birds are represented in those islands. Two 

 or more species of bat, also North American forms, are, with the 

 exception of rats and mice, the only indigenous mammals. 



Dispersal of Insects. — It is a well-known fact that insects have 

 been found in nearly all parts of the world that have thus far been 

 trod by man, from the extreme limits of the Arctic and Antarctic 

 regions to the Equator, and from the level of the sea to — and con- 

 siderably above — the line of perpetual snow. Butterflies were ob- 

 served by the naturalists of the "Alert" and "Discovery" nearly 

 as far north as the eighty-third parallel of latitude; and Hum- 

 boldt met with insects on Chimborazo, at an elevation of upwards 

 of 18,000 feet. They are found in fresh and salt waters, freely 

 swimming on the surface — and at very considerable distances from 

 the mainland — as well as below it ; in hot springs, where the water 

 has attained to a moderately high temperature, and in subteiTanean 

 caves. But, while the members of this class of animals, taken col- 

 lectively, appear to be specially adapted to all the various condi- 

 tions of existence that might be imposed upon them by accidental 

 circumstances, the same does not hold for the individual members 

 composing the class. Thus, certain insects are entirely dependent 



