In his book, The Descent of Man
, a subsection titled, "Differences in the Mental Powers of the Two Sexes
" Darwin writes:
The chief distinction in the intellectual powers of the two sexes is shewn by man's attaining to a higher eminence, in whatever he takes up, than can woman ? whether requiring deep thought, reason, or imagination, or merely the use of the senses and hands. If two lists were made of the most eminent men and women in poetry, painting, sculpture, music (inclusive both of composition and performance), history, science, and philosophy, with half-a-dozen names under each subject, the two lists would not bear comparison. We may also infer, from the law of the deviation from averages, so well illustrated by Mr. Galton, in his work on Hereditary Genius, that if men are capable of a decided pre-eminence over women in many subjects, the average of mental power in man must be above that of woman." The Descent of Man (1871) Volume II, p. 327.
After this paragraph, Darwin goes on to explain how and why this came to be (through the means of "Natural Selection" of course). Then shortly after he says (pp. 328-329):
Thus, man has ultimately become superior to woman. It is, indeed, fortunate that the law of the equal transmission of characters to both sexes prevails with mammals; otherwise, it is probable that man would have become as superior in mental endowment to woman, as the peacock is in ornamental plumage to the peahen.
In other words, had the character of the male parent not been transmitted to the female child, along with that of the female parent, man would have continued to be more and more mentally superior to the woman until his superiority would become like the superiority in appearance of the peacock (images) over the peahen (images).