ll proportion of dominants and one
containing a similar proportion of recessives are equally stable. The term
dominant is in some respects apt to be misleading, for a dominant character
cannot in virtue of its dominance establish itself at the expense of a
recessive one. Brown eyes in man are dominant to blue, but there is no
reason to suppose that as years go on the population of these islands will
become increasingly brown eyed. Given equality of conditions both are on an
equal footing. If, however, either dominant or recessive be favoured by
selection the conditions are altered, and it can be shown that even a small
advantage possessed by the one will rapidly lead to the elimination of the
other. Even with but a 5 per cent selection advantage in its favour it can
be shown that a rare sport will oust the normal form in a few hundred
generations. In this way we are freed from a difficulty inherent in the
older view that varieties arose through a long-continued process involving
the accumulation of very slight variations. On that view the establishing
of a new type was of necessity a very long and tedious business, involving
many thousands of generations. For this reason the biologist has been
accustomed to demand a very large supply of time, often a great deal more
than the physicist is {150} disposed to grant, and this has sometimes led
him to expostulate with the latter for cutting off the supply. On the newer
views, however, this difficulty need not arise, for we realise that the
origin and establishing of a new form may be a very much more rapid process
than has hitherto been deemed possible.

One last question with regard to evolution. How far does Mendelism help us
in connection with the problem of the origin of species? Among the plants
and animals with which we have dealt we have been able to show that
distinct differences, often considerable, in colour, size, and structure,
may be interpreted in terms of Mendelian factors. It is not unlikely that
most of the various characters which the systematist uses to mark off one
species from another, the so-called specific characters, are of this
nature. They serve as convenient labels, but are not essential to the
conception of species. A systematist who defined the wild sweet pea could
hardly fail to include in his definition such characters as the procumbent
habit, the tendrils, the form of the pollen, the shape of the flower, and
its purple colour. Yet all these and o