protoplasm of one species toxic to the gametes of the other, and
if this is so it is not impossible that we may some day be able to express
these differences in terms of Mendelian factors. The very nature of the
case makes it one of extreme difficulty for experimental investigation. At
any rate, we realise more clearly than before that the problem of species
is not one that can be resolved by the study of morphology or of
systematics. It is a problem in physiology.

* * * * *

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CHAPTER XIV

ECONOMICAL

Since heredity lies at the basis of the breeder's work, it is evident that
any contribution to a more exact knowledge of this subject must prove of
service to him, and there is no doubt that he will be able to profit by
Mendelian knowledge in the conduct of his operations. Indeed, as we shall
see later, these ideas have already led to striking results in the raising
of new and more profitable varieties. In the first place, heredity is a
question of individuals. Identity of appearance is no sure guide to
reproductive qualities. Two individuals similarly bred and
indistinguishable in outward form may nevertheless behave entirely
differently when bred from. Take, for instance, the family of sweet peas
shown on Plate IV. The F_2 generation here consists of seven distinct
types, three sorts of purples, three sorts of reds, and whites. Let us
suppose that our object is to obtain a true breeding strain of the pale
purple picotee form. Now from the proportions in which they come we know
that the dilute colour is due to the absence of the factor which
intensifies the colour. Consequently the picotee cannot throw the {154} two
deeper shades of red or purple. But it may be heterozygous for the purpling
factor when it will throw the dilute red (tinged white), or it may be
heterozygous for either or both of the two colour factors (cf. p. 44), in
which case it will throw whites. Of the picotees which come in such a
family, therefore, some will give picotees, tinged whites, and whites,
others will give picotees and tinged whites only, others will give picotees
and whites only, while others, again, and these the least numerous, will
give nothing but picotees. The new variety is already fixed in a certain
definite proportion of the plants; in this particular instance in 1 out of
every 27. All that remains to be done is to pick out these plants. Since
all the picotees look alike, whatever their