What the biologist calls the natural environment of an animal is from a
physical point of view a rather rigid combination of definite forces. It
is obvious that by a purposeful and systematic variation of these and
by the application of other forces in the laboratory, results must be
obtainable which do not appear in the natural environment. This is the
reasoning underlying the modern development of the study of the effects
of environment upon animal life. It was perhaps not the least important
of Darwin's services to science that the boldness of his conceptions
gave to the experimental biologist courage to enter upon the attempt of
controlling at will the life-phenomena of animals, and of bringing about
effects which cannot be expected in Nature.
The systematic physico-chemical analysis of the effect of outside
forces upon the form and reactions of animals is also our only means of
unravelling the mechanism of heredity beyond the scope of the Mendelian
law. The manner in which a germ-cell can force upon the adult certain
characters will not be understood until we succeed in varying
and controlling hereditary characteristics; and this can only be
accomplished on the basis of a systematic study of the effects of
chemical and physical forces upon living matter.
Owing to limitation of space this sketch is necessarily 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.