ther characters have been proved to
depend upon the presence of definite factors which can be removed by
appropriate crossing. By this means we can produce a small plant a few
inches in height with an erect habit of growth, without tendrils, with
round instead of oblong pollen, and with colourless deformed flowers quite
different {151} in appearance from those of the wild form. Such a plant
would breed perfectly true, and a botanist to whom it was presented, if
ignorant of its origin, might easily relegate it to a different genus.
Nevertheless, though so widely divergent in structure, such a plant must
yet be regarded as belonging to the species _Lathyrus odoratus_. For it
still remains fertile with the many different varieties of sweet pea. It is
not visible attributes that constitute the essential difference between one
species and another. The essential difference, whatever it may be, is that
underlying the phenomenon of sterility. The visible attributes are those
made use of by the systematist in cataloguing the different forms of animal
and plant life, for he has no other choice. But it must not be forgotten
that they are often misleading. Until they were bred together _Euralia
wahlbergi_ and _E. mima_ were regarded as perfectly valid species, and
there is little doubt that numbers of recognised species will eventually
fall to the ground in the same way as soon as we are in a position to apply
the test of breeding. Mendelism has helped us to realise that specific
characters may be but incidental to a species--that the true criterion of
what constitutes a species is sterility, and that particular form of
sterility which prevents two healthy gametes on uniting from producing a
zygote with normal powers of growth and reproduction. For there are forms
of sterility which are purely mechanical. The pollen of _Mirabilis jalapa_
cannot fertilise _M._ {152} _longiflora_, because the pollen tubes of the
former are not long enough to penetrate down to the ovules of the latter.
Hybrids can nevertheless be obtained from the reciprocal cross. Nor should
we expect offspring from a St. Bernard and a toy terrier without recourse
to artificial fertilisation. Or sterility may be due to pathological causes
which prevent the gametes from meeting one another in a healthy state. But
in most cases it is probable that the sterility is due to some other cause.
It is not inconceivable that definite differences in chemical composition
render the