2. The hybrids of such plants must, during the flowering period, be
protected from the influence of all foreign pollen, or be easily
capable of such protection.
The hybrids and their offspring should suffer no marked disturbance in
their fertility in the successive generations.
Accidental impregnation by foreign pollen, if it occurred during the
experiments and were not recognized, would lead to entirely erroneous
conclusions. Reduced fertility or entire sterility of certain forms,
such as occurs in the offspring of many hybrids, would render the
experiments very difficult or entirely frustrate them. In order to
discover the relations in which the hybrid forms stand towards each
other and also towards their progenitors it appears to be necessary
that all members of the series developed in each successive generation
should be, _without exception_, subjected to observation.
At the very outset special attention was devoted to the _Leguminosæ_
on account of their peculiar floral structure. Experiments which were
made with several members of this family led to the result that the
genus _Pisum_ was found to possess the necessary conditions.
Some thoroughly distinct forms of this genus possess characters which
are constant, and easily and certainly recognisable, and when their
hybrids are mutually crossed they yield perfectly fertile progeny.
Furthermore, a disturbance through foreign pollen cannot easily occur,
since the fertilising organs are closely packed inside the keel and
the anther bursts within the bud, so that the stigma becomes covered
with pollen even before the flower opens. This circumstance is of
especial importance. As additional advantages worth mentioning, there
may be cited the easy culture of these plants in the open ground and
in pots, and also their relatively short period of growth. Artificial
fertilisation is certainly a somewhat elaborate process, but nearly
always succeeds. For this purpose the bud is opened before it is
perfectly developed, the keel is removed, and each stamen carefully
extracted by means of forceps, after which the stigma can at once be
dusted over with the foreign pollen.
In all, thirty-four more or less distinct varieties of Peas were
obtained from several seedsmen and subjected to a two years’ trial. In
the case of one variety there were remarked, among a larger number of
plants all alike, a few forms which were markedly different. These,
however, did not vary in the following year, and agreed entirely
with another variety obtained from the same seedsmen; the seeds
were therefore doubtless merely accidentally mixed. All the other
varieties yielded perfectly constant and similar offspring; at any
rate, no essential difference was observed during two trial years. For
fertilisation twenty-two of these were selected and cultivated during
the whole period of the experiments. They remained constant without
any exception.
Their systematic classification is difficult and uncertain. If we
adopt the strictest definition of a species, according to which only
those individuals belong to a species which under precisely the same
circumstances display precisely similar characters, no two of these
varieties could be referred to one species. According to the opinion of
experts, however, the majority belong to the species _Pisum sativum_;
while the rest are regarded and classed, some as sub-species of _P.
sativum_, and some as independent species, such as _P. quadratum_, _P.
saccharatum_, and _P. umbellatum_. The positions, however, which may be
assigned to them in a classificatory system are quite immaterial for
the purposes of the experiments in question. It has so far been found
to be just as impossible to draw a sharp line between the hybrids of
species and varieties as between species and varieties themselves.