CHAP. II THE ELEMENTARY FACTS OF DISTRIBUTION 17 



more are restricted to the east, the centre, or the west, 

 respectively. 



Generic Areas, — Having thus obtained a tolerably clear 

 idea of the main facts as to the distribution of isolated 

 species, let us now consider those collections of closely- 

 allied species termed genera. What a genus is will be 

 sufficiently understood by a few illustrations. All the 

 different kinds of dogs, jackals, and 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 hun- 

 dred 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 



^ Many of these large genera are now subdivided, the divisions being 

 sometimes termed genera, sometimes sub-genera. 



