The term endocrine dominants brings up the inquiries of Mendelism, and the relation of Mendelian conceptions of dominant and recessive to the internal secretions.  The Mendelians have emphasized the role of the unit factor in heredity, and the conservation of the unit factor as an entity through all the adventures of matings.  Also, that when unit factors, say of the color of the eyes, come into conflict, brown or black being mixed with blue or grey, one, the recessive, is submerged and overlaid but not destroyed by the other, the dominant.  So brown or black eyes, dark hair, curly hair, dark skin, and so on, are dominant, while blue or grey eyes, light or straight hair, light skin are recessives.  A nervous temperament is dominant to the phlegmatic.  A number of psychic qualities have been declared to be Mendelian unit factors:  memory, mechanical instinct, mathematical ability, literary ability, musical ability, and even handwriting.

As architects of human qualities the endocrines must be involved in the Mendelian unit factors.  Moreover, they seem to act upon a particular locale in different degrees, which is the strongest argument against the resolution of a number of structural traits into Mendelian unit characters.  Most characters, somatic or psychic, are the products not of the action of one internal secretion alone, but of the interlinked activities of all of them.  The amount of fat deposited under the skin, for instance, is influenced by the pituitary, the thyroid, the pancreas, the liver, the adrenals and the sex glands.  Other qualities, likewise, are resultants of a compromise between all the endocrine factors comprising the equation of the individual.  If we are to look for unit factors at all in endocrine heredity, we must look more deeply into constitution, and measure the hormone potentials and their mobilization or suppression.

It will, in all probability, be found that the stability or instability of an endocrine will have a good deal to do with the part played by it in inheritance as well as in the life of the individual An unstable pituitocentric marrying another unstable pituitocentric will have children either exceptionally small or tall, or abnormally bright or stupid.  The instability tends to right itself in the next generation, or that following.  Genius as a sport, as well as sudden degeneration of family stock, the whole problem of mutation, may be closely connected with this tendency.

It has been noted that the extinction of species has been preceded by a great increase in their size, for example, the case of the great reptilia of prehistoric time.  That possibly represented pituitary stabilization, and so an abeyance of the ability to vary, necessary for fresh adaptation to a changing environment.  Indeed, endocrine instability appears the fundamental condition of the tendency to vary, endocrine stability the opposite.