is of their
definitions. Thus, Gold is a metal of high specific gravity, atomic
weight 197.2, high melting point, low chemical affinities, great
ductility, yellow colour, etc.: a Horse has 'a vertebral column,
mammae, a placental embryo, four legs, a single well-developed toe in
each foot provided with a hoof, a bushy tail, and callosities on the
inner sides of both the fore and the hind legs' (Huxley).

Since Darwinism has obtained general acceptance, some Logicians have
doubted the propriety of calling the organic species 'Kinds,' on the
ground that they are not, as to definiteness and permanence, on a par
with the chemical elements or such compounds as water and rock-salt;
that they vary extensively, and that it is only by the loss of former
generations of animals that we are able to distinguish species at all.
But to this it may be replied that species are often approximately
constant for immense periods of time, and may be called permanent in
comparison with human generations; and that, although the leading
principles of Logic are perhaps eternal truths, yet upon a detail such
as this, the science may condescend to recognise a distinction if it is
good for (say) only 100,000 years. That if former generations of plants
and animals were not lost, all distinctions of species would disappear,
may be true; but they are lost--for the most part beyond hope of
recovery; and accordingly the distinction of species is still
recognised; although there are cases, chiefly at the lower stages of
organisation, in which so many varieties occur as to make adjacent
species almost or quite indistinguishable. So far as species are
recognised, then, they present a complex co-inherence of qualities,
which is, in one aspect, a logical problem; and, in another, a logical
datum; and, coming more naturally under the head of Natural Kinds than
any other, they must be mentioned in this place.

(3) There are, again, certain coincidences of qualities not essential to
any kind, and sometimes prevailing amongst many different kinds: such as
'Insects of nauseous taste have vivid (warning) colours'; 'White
tom-cats with blue eyes are deaf'; 'White spots and patches, when they
appear in domestic animals, are most frequent on the left side.'

(4) Finally, there may be constancy of relative position, as of sides
and angles in Geometry; and also among concrete things (at least for
long periods of time), as of the planetary orbits, the apparent
positi