years or, in the course of a few days, to die after an enormous
production of asexual or sexual cells. In some instances even an almost
complete stoppage of growth may be caused, reproductive cells being
scarcely formed before the organism is again compelled to resort to
reproduction. Thus the sequence of the different stages in development
can be modified as we may desire.

The result of a more thorough investigation of the determining
conditions appears to produce at first sight a confused impression
of all sorts of possibilities. Even closely allied species exhibit
differences in regard to the connection between their development and
external conditions. It is especially noteworthy that the same form in
development may be produced as the result of very different alterations
in the environment. At the same time we can undoubtedly detect a certain
unity in the multiplicity of the individual phenomena.

If we compare the factors essential for the different stages in
development, we see that the question always resolves itself into one
of modification of similar conditions common to all life-processes. We
should rather have inferred that there exist specific external stimuli
for each developmental stage, for instance, certain chemical agencies.
Experiments hitherto made support the conclusion that QUANTITATIVE
alterations in the general conditions of life produce different types of
development. An alga or a fungus grows so long as all the conditions
of nutrition remain at a certain optimum for growth. In order to bring
about asexual reproduction, e.g. the formation of zoospores, it is
sometimes necessary to increase the degree of intensity of external
factors; sometimes, on the other hand, these must be reduced in
intensity. In the case of many algae a decrease in light-intensity or
in the amount of salts in the culture solution, or in the temperature,
induces asexual reproduction, while in others, on the contrary, an
increase in regard to each of these factors is required to produce
the same result. This holds good for the quantitative variations which
induce sexual reproduction in algae. The controlling factor is found to
be a reduction in the supply of nutritive salts and the exposure of the
plants to prolonged illumination or, better still, an increase in the
intensity of the light, the efficiency of illumination depending on the
consequent formation of organic substances such as carbohydrates.

The quantitative al