lt of their
labors is observable in the marked superiority of the breeds of the
present day over their ancestors in the last century. The improvement
of animals designed as food for man is effected by keeping them on a
liberal dietary, by selecting only the best individuals for sires and
dams, and by combining the excellencies of two or more varieties of a
species in one breed. A species consists of a number of animals which
exhibit so many points of resemblance, that they are regarded by the
great majority of naturalists to be the descendants of a single pair.
If we except the believers in the hypotheses relative to the origin
of existing varieties of animals and plants, propounded by Lamarck,
Darwin, and other naturalists of the "advanced school," there is a
general belief in the immutability of species. The individuals of an
existing species, say dogs, can never acquire the peculiar features
of another species; nor can their descendants, if we except hybrids,
ever become animals in which the characteristics of the dog tribe are
irrecognisable. By various influences, such as, for example, differences
in food and climate, and domestication, a species may be split into
_varieties_, or _breeds_, all of which, however, retain the more
important characteristics of the primordial type. There appears to be
no limit to the varieties of dogs, yet one can perceive by a glance that
there is no specific difference between the huge Mont St. Bernard dog
and the diminutive poodle, or between the sparse greyhound and the burly
mastiff. All the varieties of our domestic fowl have been traced to
a common origin--the wild Indian fowl (_Gallus bankiva_). Even Darwin
admits that all the existing kinds of horses are, in all probability,
the descendants of an original stock; and it is generally agreed that
the scores of varieties of pigeons own a common ancestor in the rock
pigeon (_Columba livia_).

As certain individuals are grouped by naturalists into species, so
particular species, which in habits and general appearance resemble each
other, are arranged under the head of genus. The horse, the ass, and the
zebra are formed on nearly the same anatomical plan; they are therefore
classed together, and designated the genus _Equus_, a term derived from
the Latin word _equus_, a horse--that animal being regarded as the type,
or perfect member of the group. Thus the horse, in the nomenclature of
the naturalist, is termed _Equus caballus_; the a