DISTRIBUTION 309 



The Holarctic realm comprises the whole of Europe, Northern 

 Africa as far south as the Sahara, Asia down to the Himalayas, and North 

 America down to Mexico. Though the faunae of all these areas are 

 fundamentally alike (as Merriam and other authorities maintain), it is 

 often convenient to divide the Holarctic into two parts: the Pal&arctic, 

 including Europe and most of temperate Asia, being limited roughly by 

 the Tropic of Cancer; and the Xearctic, occupying almost the entire 

 continent of North America, including Greenland. The northern portion 

 of the Holarctic realm forms a circumpolar belt with a remarkably homo- 

 geneous fauna and flora; therefore some authors distinguish an Arctic 

 realm, limited by the isotherm of 32, which marks very closely the tree- 

 limit. 



The boreal insects of Eurasia and North America are strikingly 

 alike. Dr. Hamilton has catalogued almost six hundred species of 

 beetles as being holarctic in distribution ; five hundred of these are com- 

 mon to Europe, Asia and North America, and the remainder are known 

 to occur in North America and also in Europe or Asia; one hundred are 

 cosmopolitan or sub-cosmopolitan, to be sure, but fifty of these are 

 probably holarctic in origin, for example Dermestes lardarius and Tene- 

 brio molitor. Of butterflies, out of some two hundred and fifty species 

 that are found in the United States east of the Rocky Mountains, scarcely 

 more than a dozen occur also in the old world. North of the United 

 States, however, as Scudder finds, no less than thirteen genera are repre- 

 sented in the old world by the same or by allied species. 



The Neotropical realm embraces South America, Central America, 

 the West Indies and the coasts of Mexico; Mexico being for the most 

 part a transition tract between the Neotropical and the Nearctic. The 

 richest butterfly fauna in the world is found in tropical South America. 

 To this region are restricted, almost without exception, the Euplceinae 

 and Lemoniinae and over ninety-nine per cent, of the Libytheinas; here 

 the Heliconiidse and Papilionidae attain their highest development, as 

 do also the Cerambycidae, or longicorn beetles. 



The Ethiopian realm consists of Africa south of the Sahara, Southern 

 Arabia and Madagascar; though some prefer to regard Madagascar as 

 a distinct realm, the Lemurian. According to Wallace, the Ethiopian 

 realm has seventy-five peculiar genera of Carabidas and is marvelously 

 rich in Cetoniidae and Lycaenidae. 



The Oriental realm includes India, Ceylon, Tropical China, and the 

 Western Malay Islands. In the richness of its insect fauna, this realm 

 vies with the Neotropical. Danaidae and Papilionidae are abundant, 



