g up the Difference; because the properties of the genus may be
found by turning to the definition of the genus; and, on the principle
of economy, whatever it is enough to do it is right to do. To define 'by
genus and difference' is a point of elegance, when the genus is known;
but the only way of knowing it is to compare the individuals comprised
in it and in co-ordinate genera, according to the methods of scientific
classification. It may be added that, as the genus represents ancestral
derivation, the predication of genus in a definition indicates the
remote causes of the phenomena denoted by the name defined. And this way
of defining corresponds with the method of double naming by genus and
species: _Felis leo_, _Felis tigris_, etc.; _Vanessa Atalanta_,
_Vanessa Io_, etc.

The so-called Genetic Definition, chiefly used in Mathematics, is a rule
for constructing that which a name denotes, in such a way as to ensure
its possessing the tributes connoted by the name. Thus, for a circle:
Take any point and, at any constant distance from it, trace a line
returning into itself. In Chemistry a genetic definition of any compound
might be given in the form of directions for the requisite synthesis of
elements.

Sec. 6. The chief difficulty in the definition of scientific names consists
in determining exactly the nature of the things denoted by them, as in
classifying plants and animals. If organic species are free growths,
continually changing, however gradually, according as circumstances give
some advantage to one form over others, we may expect to find such
species branching into varieties, which differ considerably from one
another in some respects, though not enough to constitute distinct
species. This is the case; and, consequently, there arises some
uncertainty in collecting from all the varieties those attributes which
are common to the species as a whole; and, therefore, of course,
uncertainty in defining the species. The same difficulty may occur in
defining a genus, on account of the extent to which some of its species
differ from others, whilst having enough of the common character to
deter the classifier from forming a distinct genus on their account. On
the other hand the occurrence of numerous intermediate varieties may
make it difficult to distinguish genera or species at all. Even the
Kingdoms of plants and animals are hard to discriminate at the lowest
levels of organisation. Now, where there is a difficult