of development, to understand how the simple fertilised ovum is able by
cell-division and differentiation to develop into a complicated organism
with organs and characters predetermined in the single cell which
constitutes the ovum. If we accept the idea that characters are
represented by particular parts of the chromosomes, according to Morgan's
scheme, our theory of development is the modern form of the theory of
preformation. When in the course of development the cells of the head from
which the antlers arise are formed, each of these cells must be supposed
to contain the same chromosomes as the original ovum from which the cells
have descended by repeated cell-division. The factors in these chromosomes
corresponding to the forehead have been stimulated while in the parent
animal by hormones from the outgrowth of tissue produced by external
mechanical stimulation, while at the same time they were permeated by the
testicular hormone produced either by the gametocytes themselves or by
interstitial cells of the testis. When the head begins to form in the
process of individual development, the factors, according to my theory,
have a tendency to form the special growth of tissue of which the
incipient antler consists, but part of the stimulus is wanting, and is not
completed until the testicular hormone is produced and diffused into the
circulation--that is to say, when the testes are becoming mature and
functional.

I do not claim that this theory in complete--it is impossible to
understand the process completely in the present state of knowledge--but I
maintain that it is the only theory which affords any explanation of the
remarkable facts concerning the influence of the hormones from the
reproductive organs on the development of secondary sexual characters,
while at the same time explaining the adaptive relation of these
characters or organs to the sexual habits of the various species. On the
mutation hypothesis, adaptation is purely accidental. T. H. Morgan