Mr. Wallace wrote (March 19th, 1868): "I am glad you have got good
materials on sexual selection. It is no doubt a difficult subject. One
difficulty to me is, that I do not see how the constant MINUTE variations,
which are sufficient for Natural Selection to work with, could be SEXUALLY
selected. We seem to require a series of bold and abrupt variations. How
can we imagine that an inch in the tail of the peacock, or 1/4-inch in that
of the Bird of Paradise, would be noticed and preferred by the female.")

In regard to sexual selection. A girl sees a handsome man, and without
observing whether his nose or whiskers are the tenth of an inch longer or
shorter than in some other man, admires his appearance and says she will
marry him. So, I suppose, with the pea-hen; and the tail has been
increased in length merely by, on the whole, presenting a more gorgeous
appearance. J. Jenner Weir, however, has given me some facts showing that
birds apparently admire details of plumage.


LETTER 432. TO F. MULLER.
March 28th [1868].

I am particularly obliged to you for your observations on the stridulation
of the two sexes of Lamellicorns. (432/1. We are unable to find any
mention of F. Muller's observations on this point; but the reference is
clearly to Darwin's observations on Necrophorus and Pelobius, in which the
stridulating rasp was bigger in the males in the first individuals
examined, but not so in succeeding specimens. "Descent of Man," Edition
II., Volume I., page 382.) I begin to fear that I am completely in error
owing to that common cause, viz. mistaking at first individual variability
for sexual difference.