dification, may be treated in a short summary which falls under
two heads, one having reference to the conditions of form-production in
single species, the other being concerned with the conditions governing
the transformation of species.

I. THE INFLUENCE OF EXTERNAL CONDITIONS ON FORM-PRODUCTION IN SINGLE
SPECIES.

The members of plants, which we express by the terms stem, leaf, flower,
etc. are capable of modification within certain limits; since Lamarck's
time this power of modification has been brought more or less into
relation with the environment. We are concerned not only with the
question of experimental demonstration of this relationship, but, more
generally, with an examination of the origin of forms, the sequences of
stages in development that are governed by recognisable causes. We have
to consider the general problem; to study the conditions of all typical
as well as of atypic forms, in other words, to found a physiology of
form.

If we survey the endless variety of plant-forms and consider the highly
complex and still little known processes in the interior of cells, and
if we remember that the whole of this branch of investigation came into
existence only a few decades ago, we are able to grasp the fact that
a satisfactory explanation of the factors determining form cannot be
discovered all at once. The goal is still far away. We are not concerned
now with the controversial question, whether, on the whole, the
fundamental processes in the development of form can be recognised by
physiological means. A belief in the possibility of this can in any case
do no harm. What we may and must attempt is this--to discover points
of attack on one side or another, which may enable us by means of
experimental methods to come into closer touch with these elusive
and difficult problems. While we are forced to admit that there is at
present much that is insoluble there remains an inexhaustible supply of
problems capable of solution.

The object of our investigations is the species; but as regards the
question, what is a species, science of to-day takes up a position
different from that of Darwin. For him it was the Linnean species which
illustrates variation: we now know, thanks to the work of Jordan,
de Bary, and particularly to that of de Vries (de Vries, "Die
Mutationstheorie", Leipzig, 1901, Vol. I. page 33.), that the Linnean
species consists of a large or small number of entities, elementary
species. In experi