Hence pedigree-book-keeping must become the general rule. Subordinate to
this are two further points, which should likewise be stated here. One
pertains to the pure or hybrid nature of the original strain, and the
other to the life-conditions and all other external influences. It is
manifest that a complete understanding of a mutation depends upon full
information upon these points.

All experiments must have a beginning. The starting-point may be a
single individual, or a small group of plants, or a lot of seeds. In
many cases the whole previous history is obscure, but sometimes a little
historical evidence is at hand. Often it is evident that the initial
material belongs to a pure species, but with respect to the question of
elementary species it is [25] not rarely open to doubt. Large numbers of
hybrid plants and hybrid races are in existence, concerning the origin
of which it is impossible to decide. It is impossible in many instances
to ascertain whether they are of hybrid or of pure origin. Often there
is only one way of determining the matter; it is to guess at the
probable parents in case of a cross and to repeat the cross. This is a
point which always requires great care in the interpretation of unusual
facts.

Three cases are to be distinguished as to heredity. Many plants are so
constituted as to be fertilized with their own pollen. In this case the
visits of insects have simply to be excluded, which may be done by
covering plants with iron gauze or with bags of prepared paper.
Sometimes they fertilize themselves without any aid, as for instance,
the common evening-primrose; in other cases the pollen has to be placed
on the stigma artificially, as with Lamarck's evening-primrose and its
derivatives. Other plants need cross-fertilization in order to produce a