ion.

During long cultivation under conditions which vary in very different
degrees, such as moisture, the amount of salts, light intensity,
temperature, oxygen, it is possible that sudden and special disturbances
in the relations of the cell substances have a directive influence on
the inner organisation of the sexual cells, so that not only inconstant
but also constant varieties will be formed.

Definite proof in support of this view has not yet been furnished, and
we must admit that the question as to the cause of heredity remains,
fundamentally, as far from solution as it was in Darwin's time. As the
result of the work of many investigators, particularly de Vries,
the problem is constantly becoming clearer and more definite. The
penetration into this most difficult and therefore most interesting
problem of life and the creation by experiment of new races or
elementary species are no longer beyond the region of possibility.

XIV. EXPERIMENTAL STUDY OF THE INFLUENCE OF ENVIRONMENT ON ANIMALS.
By Jacques Loeb, M.D. Professor of Physiology in the University of
California.

I. INTRODUCTORY REMARKS.

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 neces