. Entomology : with special reference to its biological and economic aspects. hough, in most cases, slight differences distin-guishing the individuals of each island as a race, from thoseinhabiting any other island. There are two and Bindloe have the same form, and Albemarlesupports at least two races. Each of these two species pre-sents no less than five racial types, to which distinctive nameshave been applied. Though the relationships and evolution ofthese races have been ably discussed by Snodgrass, definiteconclusions upon these subjects are still needed. Isolation ing
. Entomology : with special reference to its biological and economic aspects. hough, in most cases, slight differences distin-guishing the individuals of each island as a race, from thoseinhabiting any other island. There are two and Bindloe have the same form, and Albemarlesupports at least two races. Each of these two species pre-sents no less than five racial types, to which distinctive nameshave been applied. Though the relationships and evolution ofthese races have been ably discussed by Snodgrass, definiteconclusions upon these subjects are still needed. Isolation ingeneral we have considered briefly in Chapter VII. Faunal Realms.—The general distribution of life is suchthat naturalists divide the earth into several realms, each ofwhich has its characteristic fauna and flora. As to the preciseboundaries of these faunal realms, zoologists do not all agree,owing chiefly to the fact that faunje overlap one another tosuch an extent as to render their exact separation more or lessarbitrary. Five realms, at least, are generally recognized:. DISTRIBUTION 3/5 Holarctic, Neofroj^ical, EtJiiopiaii, Oriental and AiistraHcni (PI. 3)- 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 ]\Iexico. Though thefaunae of all these areas are fundamentally alike (as Merriamand other authorities maintain), it is often convenient todivide the Holarctic into two parts: the Palcearctic, includingEurope and most of temperate Asia, being limited roughly bythe Tropic of Cancer; and the Nearctic, occupying almost theentire continent of North America, including Greenland. Thenorthern portion of the Holarctic realm forms a circumpolarbelt with a remarkable homogeneous fauna and flora; there-fore some authors distinguish an Arctic realm, limited by theisotherm of T)-^^ which marks very closely the tree-limit. The boreal insects of Eurasia and North America are strik-ingly
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