he first is said to have been
introduced from Virginia, and the second from Canada, but both probably
from plants cultivated in the gardens of these countries. Whether the
same elementary species are still growing on those spots is unknown,
mainly because the different sub-species of the species mentioned have
not been systematically studied and distinguished.

The origin of new species, which is in part the effect of mutability,
is, however, due mainly to natural selection. Mutability provides the
new characters and new elementary species. Natural selection, on the
other hand, decides what is to live and what to die. Mutability seems to
be free, and not restricted to previously determined lines. Selection,
however, may take place along the same main lines in the course of long
geological epochs, thus directing the development of large branches of
the animal and vegetable kingdom. In natural selection it is evident
that nutrition and environment are the main factors. But it is probable
that, while nutrition may be one of the main causes of mutability,
environment may play the chief part in the decisions ascribed to natural
selection. Relations to neighbouring plants and to injurious or useful
animals, have been considered the most important determining factors
ever since the time when Darwin pointed out their prevailing influence.

From this discussion of the main causes of variability we may derive the
proposition that the study of every phenomenon in the field of heredity,
of variability, and of the origin of new species will have to be
considered from two standpoints; on one hand we have the internal
causes, on the other the external ones. Sometimes the first are more
easily detected, in other cases the latter are more accessible to
investigation. But the complete elucidation of any phenomenon of life
must always combine the study of the influence of internal with that of
external causes.

III. POLYMORPHIC VARIABILITY IN CEREALS.

One of the propositions of Darwin's theory of the struggle for life
maintains that the largest amount of life can be supported on any area,
by great diversification or divergence in the structure and constitution
of its inhabitants. Every meadow and every forest affords a proof of
this thesis. The numerical proportion of the different species of the
flora is always changing according to external influences. Thus, in a
given meadow, some species will flower abundantly in one year and then