state, whether we are dealing with a single cell or with a
small group of cells. These have been produced from pre-existing cells
and they in turn from others; the problem is constantly pushed back
through a succession of generations until it becomes identified with
that of the origin of species.

A way, however, is opened for investigation; experience teaches us that
this inner world is not a constant factor: on the contrary, it appears
to be very variable. The dependence of VARIABLE INTERNAL on VARIABLE
EXTERNAL conditions gives us the key with which research may open the
door. In the lower plants this dependence is at once apparent, each cell
is directly subject to external influences. In the higher plants with
their different organs, these influences were transmitted to cells in
course of development along exceedingly complex lines. In the case of
the growing-point of a bud, which is capable of producing a complete
plant, direct influences play a much less important part than those
exerted through other organs, particularly through the roots and leaves,
which are essential in nutrition. These correlations, as we may call
them, are of the greatest importance as aids to an understanding of
form-production. When a bud is produced on a particular part of a plant,
it undergoes definite internal modifications induced by the influence of
other organs, the activity of which is governed by the environment, and
as the result of this it develops along a certain direction; it may,
for example, become a flower. The particular direction of development
is determined before the rudiment is differentiated and is exerted so
strongly that further development ensues without interruption, even
though the external conditions vary considerably and exert a positively
inimical influence: this produces the impression that development
proceeds entirely independently of the outer world. The widespread
belief that such independence exists is very premature and at all events
unproven.

The state of the young rudiment is the outcome of previous influences of
the external world communicated through other organs. Experiments show
that in certain cases, if the efficiency of roots and leaves as organs
concerned with nutrition is interfered with, the production of flowers
is affected, and their characters, which are normally very constant,
undergo far-reaching modifications. To find the right moment at which to
make the necessary alteration in the en