The same endocrine constitutions will produce corresponding physiques, physiognomies, abilities and characters.  Deviations in endocrine type from that of the original stock, more of one endocrine and less of another, is at the bottom of the phenomenon of variation, basic for the origin of new species as well as the extinction of the old.  In short, viewing the internal secretions as determinants, by their quantitative variations, of a host of biologic phenomena furnishes a concrete and detailed foundation for Darwin’s theory of pangenesis.

INHERITANCE OF ACQUIRED CHARACTERS

Darwin’s theory of pangenesis was an attempt to harmonize everything known in his time about heredity.  It supposed that the various organs of the body gave off into the blood substances, themselves in miniature, which were taken up by the sex cells, and so became responsible for the development of their mother-organ in the newly forming individual.  Modern knowledge cannot accept all this as a whole.  But in a modified version, it has become the germ of a theory of heredity of which J.T.  Cunningham, of Oxford, is the chief backer.

Beginning with the traits and qualities which distinguish the sexes, grouped as the secondary sex characters, he showed that they are correlated with the special sexual function of the species in which they occur.  These traits appear only when the hormones occur which are present in one sex and that only when the gonads of that sex are mature.  In some cases they appear only at the period of the year when reproduction takes place, disappearing again after the breeding season.  Their presence makes certain cells develop in excessive numbers at a particular spot in the organism (as in the growth of breasts from a few sweat glands) or causes them to specialize (to make hair on the face in man, or to grow antlers on the head of a stag).  After castration, the hormones being absent, all these points of contrast between the sexes fail to appear.  So by analogy we may explain all somatic and psychic differentiation as functions of the glands of internal secretion.  Contemplated from the angle of the effect of environment upon the endocrines, and a reflected action upon the germ cells, we may outline a mechanism of the inheritance of acquired characters at certain times and consequent adaptation.  The cycle of events would be as follows: 

1.  A state of lability of cells at a point because of increased or decreased use.

2.  An increased or decreased appropriation by them of the hormone controlling their function.

3.  A corresponding increase or decrease in function of the gland of internal secretion and so,

4.  An increased or decreased representation of it in the reproductive sex cells in the gonads.