whatever they are they have never undergone any change in their
distinguishing characters. At the same time we know that whether we call
related forms varieties or species or genera in different cases, we find,
whatever organisms we study, whether plants or animals, definite types
distinguished by special characters of form, colour, and structure, and
that individuals of one species or type never give rise by generation to
individuals of any other known species or type. We do not find wolves
producing foxes, or bulldogs giving birth to greyhounds. As a general
rule the distinguishing characters are inherited, and it is by no means
easy even in domesticated animals and plants to obtain an exact and
complete record of the descent of a new variety from the original form.
Among species in a state of nature it is the exception to find two
recognised species which can be crossed or hybridised. In the case of the
horse and the ass, although mules are the hybrid offspring of the two, the
mules themselves are sterile, and there are many similar cases, so that
some naturalists have maintained that mutual infertility should be
recognised as the test of separation in species.

Darwin founded his theory on the assumption that differences of species
were differences of adaptation. His theory of natural selection is a
theory of the origin of adaptations, and only a theory of the origin of
species on the assumption that their distinguishing characters are
adaptations to different modes and conditions of life, to different
requirements. He pointed out that there is always a considerable range of
variation in the specific characters, that, as a rule, no two individuals
are exactly alike, even when produced by the same two parents. The central
principle of his theory was the survival of individuals possessing those
variations which were most useful in the competition of species with
species and of individual with individual. He thus explained adaptation to
new conditions and divergence of several species from a common ancestor.