t DISTRIBUTION 331 



aunae overlap one another to such an extent as to render their exact 

 eparation more or less arbitrary. Five realms, at least, are generally 

 ecognized: Holarctic, Neotropical, Ethiopian, Oriental and Australian 

 Plate III). 

 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 

 Palaarctic, including Europe and most of temperate Asia, being limited 

 roughly by the Tropic of Cancer; and the Nearctic, occupying almost 

 the entire continent of North America, including Greenland. The 

 northern portion of the Holarctic realm forms a circumpolar belt with a 

 remarkably homogeneous 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 (larder 

 beetle) and Tenebrio molitor (meal-worm). 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 

 fewer than thirteen genera are represented 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 Libytheinae; here 

 the Heliconiidae 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 



