enus 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.[5]

_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 {18} each other, each species occupying an area of its own which
rarely coincides 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, while allied species
come to the north bank, which in like manner forms their boundary. As
examples we may mention that one of the Saki monkeys (_Pithecia monachus?_)
comes up to the south bank of the Upper Amazon, while immediately we cross
over to the north bank we find another species (_Pithecia rufibarbata?_).
Among birds we have the green jacamar (_Galbula viridis_), abundant on the
north bank of the Lower Amazon, while on the south bank we have two allied
species (_Galbula rufoviridis_ and _G. cyaneicollis_); and among insects we
have at Santarem on the south bank of the Amazon, the beautiful blue
butterfly, _Callithea sapphira_, while almost opposite to it, at
Monte-alegre, an allied species,