vironment is indeed difficult
and in many cases not yet possible. This is especially the case with
fertilised eggs, which in a higher degree than buds have acquired,
through parental influences, an apparently fixed internal organisation,
and this seems to have pre-determined their development. It is, however,
highly probable that it will be possible, by influencing the parents,
to alter the internal organisation and to switch off development on to
other lines.

Having made these general observations I will now cite a few of the many
facts at our disposal, in order to illustrate the methods and aim of the
experimental methods of research. As a matter of convenience I will deal
separately with modification of development and with modification of
single organs.

I. EFFECT OF ENVIRONMENT UPON THE COURSE OF DEVELOPMENT.

Every plant, whether an alga or a flowering plant passes, under natural
conditions, through a series of developmental stages characteristic of
each species, and these consist in a regular sequence of definite
forms. It is impossible to form an opinion from mere observation and
description as to what inner changes are essential for the production of
the several forms. We must endeavour to influence the inner factors by
known external conditions in such a way that the individual stages in
development are separately controlled and the order of their sequence
determined at will by experimental treatment. Such control over the
course of development may be gained with special certainty in the case
of the lower organisms.

With these it is practicable to control the principal conditions of
cultivation and to vary them in various ways. By this means it has been
demonstrated that each developmental stage depends upon special external
conditions, and in cases where our knowledge is sufficient, a particular
stage may be obtained at will. In the Green Algae (See Klebs, "Die
Bedingung der Fortpflanzung... ", Jena, 1896; also "Jahrb. fur
Wiss. Bot." 1898 and 1900; "Probleme der Entwickelung, III." "Biol.
Centralbl." 1904, page 452.), as in the case of Fungi, we may classify
the stages of development into purely vegetative growth (growth,
cell-division, branching), asexual reproduction (formation of zoospores,
conidia) and sexual processes (formation of male and female sexual
organs). By modifying the external conditions it is possible to induce
algae or fungi (Vaucheria, Saprolegnia) to grow continuously for
several