CHAP. II.] THE ELEMENTARY FACTS OF DISTRIBUTION. 17 



wolves belong to the dog genus, Canis ; the tiger, lion, leopard, 

 jaguar, and the wild cats, to the cat genus, Felis; the blackbird, 

 song-thrush, missel-thrush, fieldfare, and many others, to the 

 thrush genus, Turdus; the crow, rook, raven, and jackdaw, 

 to the crow genus, Corvus ; but the magpie belongs to another, 

 though closely-allied genus. Pica, distinguished by the different 

 form and proportions of its wings and tail from all the species 

 of the crow genus. The number of species in a genus varies 

 greatly, from one up to several hundreds. The giraffe, the 

 glutton, the walrus, the bearded reedling, the secretary-bird, 

 and many others, have no close allies, and each forms a 

 genus by itself. The beaver genus, Castor, and the camel 

 genus, Camelus, each consist of two species. On the other 

 hand, the deer genus, Cervus has forty species ; the mouse 

 and rat genus, Mus more than a hundred species ; and there 

 is about the same number of the thrush genus ; while among 

 the lower classes of animals genera are often very extensive, 

 the fine genus Papilio, or swallow-tailed butterflies, containing 

 more than four hundred species; and Cicindela, which includes 

 our native tiger beetles, has about the same number. Many 

 genera of shells are very extensive, and one of them — the 

 genus Helix, including the commonest snails, and ranging 

 all over the world — is probably the most extensive in the 

 animal kingdom, numbering about two thousand described 

 species. 



Separate and overlapping Areas. — The species of a genus are 

 distributed in two ways. Either they occupy distinct areas 

 which do not touch each other and are sometimes widely 

 separated, or they touch and occasionally overlap each other, 

 each species occupying an area of its own which rarely coin- 

 cides exactly with that of any other species of the same genus. 

 In some cases, when a river, a mountain-chain, or a change of 

 conditions as from pasture to desert or forest, determines the 

 range of species, the areas of two species of the same genus 

 may just meet, one beginning where the other ends ; but this 

 is comparatively rare. It occurs, however, in the Amazon 

 valley, where several species of monkeys, birds, and insects 

 come up to the south bank of the river but do not pass it, 



c 



