greatest
discoveries. It was Malthus who suggested to Darwin the great
principle of the struggle for existence among men which Darwin so
successfully applied to other organisms, and used so profitably in
building up his great theory of natural selection. It was from the
sociologist that the biologist derived his idea of the physiological
division of labor which has proved so fruitful a conception; and from
the same source he has drawn many of his conceptions of organic
individuality.

We might suggest here some of the topics upon which biology has
information of value in this bio-social field; many of these we shall
discuss later on from our present and special point of view. First of
all come the facts regarding the variability and variation of human
beings, not alone in physical characteristics, but in respect to
psychic traits as well. Here as in all organisms we must distinguish
between true variations and bodily modifications; that is, we must be
careful to make, as far as possible, the biological distinction
between innate and acquired traits, particularly in considering
mental characteristics. Next must come consideration of the facts of
heredity. This is undoubtedly the field of greatest importance to the
Eugenist; facts of no other kind are of equal significance in
determining the course of eugenic practice. We now have a fairly
extensive working basis here from which to discuss heredity in man.
The various phases of human selection should be noticed, in particular
that known as selective fertility or differential fertility in
different social groups or classes. Another evolutionary factor of
importance here is that of "isolation" in the many and varied forms
which it assumes in human society, especially those which result from
assortative and preferential mating, and from the operation of social
convention, restrictions in marriage, and the like.

Before discussing any of these subjects let us offer here just a word
of caution to the enthusiast. The results gained in one field of
science cannot be transferred _in toto_ to another field and there be
found to fit. Biology has learned much from Physics and Chemistry, but
the biological applications of the laws of these sciences must be
carried out with the greatest care. Such transference has often been
premature and attended by results retardative to progress in the field
of Biology. Any formula borrowed from one science and applied in
another must be rigorou