Lion.                                 Tiger.
SIZE:  about 9-1/2 feet from nose   | About 10 feet.
to tip of tail.                  |
COLOUR:  tawny.                     | Warm tawny, striped with black. 
TAIL:  tufted.                      | Tapering. 
MANE:  present in the male.         | Both sexes maneless.

There are other differences in the shape of the skull.  In defining lion, then, it would have been enough to mention the genus and the properties making 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 difficulty of classification there must be a corresponding difficulty of definition.