The Descent of Man

given, for raising the races of man to the dignity
of species, and the insuperable difficulties on
the other side in defining them, it seems that
the term "sub-species" might here be used with
propriety. But from long habit the term "race"
will perhaps always be employed. The choice
of terms is only so far important in that it is
desirable to use, as far as possible, the same
terms for the same degrees of difference. Unfortunately
this can rarely be done: for the larger
genera generally include closely- allied forms,
which can be distinguished only with much
difficulty, whilst the smaller genera within the
same family include forms that are perfectly
distinct; yet all must be ranked equally as species.
So again, species within the same large
genus by no means resemble each other to the
same degree: on the contrary, some of them can
generally be arranged in little groups round
other species, like satellites round planets. (20.
'Origin of Species,' 5th edit. p. 68.)