Sir,

Allow me a word as to a point of detail.

On p. 293 (vol 1) of “The Descent of Man”¹ you say:

On the other hand, the tortoise-shell colour of the hair, which is confined to female cats, is quite distinct at birth, and this case violates our rule,”—the rule being (p. 292)

that characters which appear late in life in one sex are transmitted exclusively to the same sex.

Now this case does not violate the rule, because it does not come under it: for of “the characters which appear late in life” the tortoise-shell colour is not one, since it is there “at birth”. It by no means is synonymous with the proposition: “Characters appearing late in life in one sex are transmitted exclusively to the same sex—that “Characters transmitted exclusively to the same sex appear late in life”. The latter is the inverse of the true rule, just as the proposition that “Characters developed early tend to be transmitted to both sexes” is (as you have shewn) the converse of it. And as that converse has been partly rejected, to sustain & expound the true principle, it would be a pity that the inverse, which seems unintentionally to have crept in, should be retained to impede unjustly the natural scope and force of a principle so definitely valuable and at a point so interesting.

(It may, or may not, be a principle generally accurate, that characters transmitted exclusively to the same sex appear late in life. But this is a separate principle, liable to its own exceptions, and to be judged on its own merits—as might also be the principle, converse to it, that characters transmissible to both sexes are developed at an early period. And it seems to me, in the discussion generally you keep to the one statement of each principle as it occurs on p. 292; for example at the head of p. 286.)

With apologies, believe me | Your obedient Servant, | William Renton.

Charles Darwin, Esq.