lves,
etc., walk upon their toes, affect the application of any of these
names.

Similarly, the extension of a name to things not previously denoted by
it, may not in fact alter its definition; for the extension may be made
on the very ground that the things now first denoted by it have been
found to have the properties enumerated in its definition, as when the
name 'mammal' was applied to whales, dolphins, etc. If, however,
'mammal' had formerly been understood to apply only to land animals, so
that its definition included (at least, popularly) the quality of
'living on the land,' this part of the connotation was of course lost
when the denotation came to include certain aquatic animals.

A proprium is an attribute derived from the definition: being either (a)
implied in it, or deducible from it, as 'having its three angles equal
to two right angles' may be proved from the definition of a triangle; or
(b) causally dependent on it, as being 'dangerous to flocks' results
from the nature of a wolf, and as 'moving in an ellipse' results from
the nature of a planet in its relation to the sun.

An accident is a property accompanying the defining attributes without
being deducible from them. The word suggests that such a property is
merely 'accidental,' or there 'by chance'; but it only means that we do
not understand the connection.

Proprium and Accident bear the same relation to one another as
Derivative and Empirical Laws: the predication of a proprium is a
derivative law, and the predication of an accident is an empirical law.
Both accidents and empirical laws present problems, the solution of
which consists in reducing them, respectively, to propria and derivative
laws. Thus the colour of animals was once regarded as an accident for
which no reason could be given; but now the colour of animals is
regarded as an effect of their nature and habits, the chief determinants
of it being the advantage of concealment; whilst in other cases, as
among brightly coloured insects and snakes, the determinant may be the
advantage of advertising their own noxiousness. If such reasoning is
sound, colour is a proprium (and if so, it cannot _logically_ be
included in a definition; but it is better to be judicious than formal).

If the colour of animals is a proprium, we must recognise a distinction
between Inseparable and Separable Propria, according as they do, or do
not, always accompany the essence: for mankind is regarded as one