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. Jenner Weir, however, [pg 205]has given me some facts showing that birds apparently admire details of plumage.—Yours most sincerely,
C. DARWIN.
Hurstpierpoint. March 24, [1868?].
Dear Darwin,—Many thanks for the photo, which I shall get when I go to town.
I return your son's notes with my notes on them.
Without going into any details, is not this a strong general argument?—
1. A species varies occasionally in two directions, but owing to their free intercrossing they (the variations) never increase.
2. A change of conditions occurs which threatens the existence of the species, but the two varieties are adapted to the changing conditions, and, if accumulated, will form two new species adapted to the new conditions.
3. Free crossing, however, renders this impossible, and so the species is in danger of extinction.