secondary sexual characters, and does not in the least explain sexual
dimorphism. In the first place, the term sex-linked does not mean
occurring always exclusively in one sex, but the direct contrary--
transmitted by one sex to the opposite sex--and in the second place there
is no suggestion that the development of the character is dependent in any
way on the presence or function of the gonad. The problem I am proposing
to consider is what light the facts throw on the origin of the secondary
sexual characters in evolution. In endeavouring to answer this question
there are only two alternatives: either the characters are blastogenic--
that is, they arise from some change in the gametocytes occurring
somewhere in the succession of cell-divisions of these cells--or they
arise in the soma and are impressed on the gametocytes by the influence of
the soma within which these gametocytes are contained--that is to say,
they are somatogenic. That characters do originate by the first of these
processes may be considered to be proved by recent researches, and such
characters are called mutations. There can be little doubt that the so-
called sex-linked characters, of which examples have been given above,
have originated in this way, and that their relation to sex is part of the
mutation. According to T. H. Morgan, it is simply due to the fact that
the determinants for such characters are situated in the sex-chromosome.
Morgan, however, also states that a case of true sexual dimorphism arose
as a mutation in his cultures of _Drosphilia_. The character was eosin
colour in the eye instead of the red colour of the eye in the original
fly. In the female this was dark eosin colour, in the male yellowish
eosin. But this case differs from the characters particularly under
consideration here in two points: (1) there is no suggestion that it was
adaptive, (2) or that it was influenced by hormones from the gonads.

No character whose development is dependent in greater or less degree on
the stimulation of some substance derived from the gonads can have