chap, il] 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 
