egnancy.
Pregnant animals fed on thyroid give birth to young with large thymus
glands. The diet of the mother has been proved conclusively to
influence the development and constitution of the child. As the
internal secretions influence the history of the food in the body,
they affect development in the womb indirectly as well as directly.
Certainly, whether or no we learn how to change the nature of
germplasm within a short time, we have in the endocrines the means at
hand for affecting _the whole individual that is born and sees the
light of day_.

THE CONTROL OF MUTATIONS

The true physical and intellectual evolution of man depends upon the
production of mutations of a desirable kind that can survive. The
information furnished by the study of the endocrines concerning
the genesis of personality provides the foundations for a positive
eugenics, a eugenics of the encouragement of desirable matings, with
the proper legal and social procedures. Selective breeding for the
production of the best endocrine types should become practicable.

But the biologist should be able to go farther. If the eugenist is to
limit himself to the method of the animal breeder he will have to rest
satisfied with the characters or hereditary factors given, that turn
up spontaneously in an individual. But with the internal secretions
as the controllable controllers of mutations, the outlook changes.
It should become possible to produce new mutations, good and bad, to
speed up their production at any rate. The feeding of thyroid to
a gifted father before procreation might enhance immeasurably the
chances of transmission of his gift as well as of its intensification
in his offspring. A field of investigation is opened that would
embrace in due time the deliberate control of human evolution.

All the physical traits, stature, color, muscle function, and so on,
offer themselves for improvement, as well as brain size, and the
intellectual and emotional factors which have dominated man's social
evolution. The general prevalence of nervous disorders in civilized
countries, visible even in the nervous infants the specialist in
children's diseases is called upon to treat, shows that the nervous
system of the better part of mankind is in a state of unstable
equilibrium. It may be another example of the curious coincidences
that have been called the Fitness of the Environment that the
investigation of the endocrines promises to put into our hands the