; and
if the material is abundant, and the distinguishing characters clear
and well defined, we conclude that there is a specific difference.

Species determined in this way must possess certain general properties
in common:

1. Their individuals must fall within a certain range of uniform
characters, wider or narrower in the case of different species.

2. The intervals between species must be distinctly marked, and not
slurred over by intermediate gradations.

3. The specific characters must be invariably transmitted from
generation to generation, so that they remain equally distinct in
their limits if traced backward or forward in time, in so far as our
observation may extend.

4. Within the limits of the species there is more or less liability to
variation; and this, though perhaps developed by external
circumstances, is really inherent in the species, and must necessarily
form a part of its proper description.

5. There is also a physiological distinction between species, namely,
that the individuals are sterile with one another, whereas this does
not apply to varieties; and though Darwin has labored to break down
this distinction by insisting on rare exceptional cases, and
suggesting many supposed ways by which varieties of the same species
might possibly attain to this kind of distinctness, the difference
still remains as a fact in nature; though one not readily available in
practically distinguishing species.

These general properties of species will, I think, be admitted by all
naturalists as based on nature, and absolutely necessary to the
existence of natural history as a science, independently of any
hypotheses as to the possible changes of specific forms in the lapse
of time. I now proceed to give a similar summary of the laws of the
varieties which may exist--always be it observed, within the limits of
the species.

1. The limits of variation are very different in different species.
There are many in which no well-marked variations have been observed.
There are others in which the variations are so marked that they have
been divided, even by skilful naturalists, into distinct species or
even genera. I do not here refer to differences of age and sex. These
in many animals are so great that nothing but actual knowledge of the
relation that subsists would prevent the individuals from being
entirely separated from one another. I refer merely to the varieties
that exist in adults of the same sex, incl