rigin of the remarkable differences between individuals that
distinguish species, varieties and families, has long been one of the
chief puzzles of biology. It may indeed be called the leading puzzle,
which led Darwin on to the collection of the data that culminated in
the "Origin of Species." The why of the Unique is the fundamental
problem of those who would understand life.

An explanation is an attempt at a consistent and persistent, sometimes
an obstinate clarity of mind. A vast number of observations gathered
by laboratory experimentalists as well as by those naturalists of the
abnormal, physicians in active practice, prove that the construction
of the individual both during development before maturity, and
maintenance during maturity, his constitution, in short, is directed
by the endocrine glands. It is possible now to present an explanation
of the individuality of the individual.

To assert that variation is responsible for the individual, that it
is the mechanism which isolates him as a being like none other of his
fellows, not even his parents, brothers, and sisters, is merely to beg
the question. What is variation? The internal secretion theory of the
process offers, for the first time, an explanation that is coherent
and comprehensive, based upon concrete and detailed observations.
It provides an adequate interpretation of the numberless hereditary
gradations and transitions, blendings and mixtures. It suggests a
control of heredity in the future.

THE PURE TYPES

In the pure types, only one gland, either by being present in great
excess above the average, or by being pretty well below the average,
comes to exercise the dominating influence upon the traits of the
organism. As the strongest link in the chain, or as the weakest, it
rules. The others must accommodate themselves to it. Among them as
commanders of growth, development and normal function, it holds the
balance of power. In every emergency it stands out by its strength or
by its weakness. It thus creates its own type of man or woman, with
attributes and characteristics peculiar to itself. These pure types,
as we have seen, are mainly the thyroid, the pituitary, and the
adrenal-centered.

Each with the signs peculiar to it can be identified among the faces
that pass one in the street. And they differ so markedly among
themselves that they provide a new and accurate means of classifying
varieties among the races of the species: man. The thyroid