DISTRIBUTION 375 
Holarctic, Neotropical, Ethiopian, Oriental and Australian 
GP es): 
The Holarctic realm comprises the whole of Europe, North- 
ern Africa as far south as the Sahara, Asia down to the Hima- 
layas, and North America down to Mexico. Though the 
faunze 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 Palearctic, 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 remarkable homogeneous fauna and flora; there- 
fore 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 strik- 
ingly alike. Dr. Hamilton has catalogued nearly six hundred 
species of beetles as being holarctic in distribution; five hun- 
dred of these are common 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 hol- 
arctic in origin, for example—Dermestes lardarius and Tene- 
brio molitor. Of butterflies, out of scme 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 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 Neo- 
tropical 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 Euplceinze and 
Lemoniinz and over ninety-nine per cent. of the Libytheine; 
