sarily very incomplete,
and it must not be inferred that studies which are not mentioned here
were considered to be of minor importance. All the writer could hope to
do was to bring together a few instances of the experimental analysis of
the effect of environment, which indicate the nature and extent of our
control over life-phenomena and which also have some relation to the
work of Darwin. In the selection of these instances preference is given
to those problems which are not too technical for the general reader.

The forces, the influence of which we shall discuss, are in succession
chemical agencies, temperature, light, and gravitation. We shall also
treat separately the effect of these forces upon form and instinctive
reactions.

II. THE EFFECTS OF CHEMICAL AGENCIES.

(a) HETEROGENEOUS HYBRIDISATION.

It was held until recently that hybridisation is not possible except
between closely related species and that even among these a successful
hybridisation cannot always be counted upon. This view was well
supported by experience. It is, for instance, well known that the
majority of marine animals lay their unfertilised eggs in the ocean and
that the males shed their sperm also into the sea-water. The numerical
excess of the spermatozoa over the ova in the sea-water is the only
guarantee that the eggs are fertilised, for the spermatozoa are
carried to the eggs by chance and are not attracted by the latter. This
statement is the result of numerous experiments by various authors,
and is contrary to common belief. As a rule all or the majority of
individuals of a species in a given region spawn on the same day, and
when this occurs the sea-water constitutes a veritable suspension of
sperm. It has been shown by experiment that in fresh sea-water the sperm
may live and retain its fertilising power for several days. It is thus
unavoidable that at certain periods more than one kind of spermatozoon
is suspended in the sea-water and it is a matter of surprise that the
most heterogeneous hybridisations do not constantly occur. The reason
for this becomes obvious if we bring together mature eggs and equally
mature and active sperm of a different family. When this is done no egg
is, as a rule, fertilised. The eggs of a sea-urchin can be fertilised by
sperm of their own species, or, though in smaller numbers, by the sperm
of other species of sea-urchins, but not by the sperm of other groups of
echinoderms, e.g. starfish, britt