(431/1. "The Variation of Animals and Plants" having been published on January 30th, 1868, Mr. Darwin notes in his diary that on February 4th he "Began on Man and Sexual Selection." He had already (in 1864 and 1867) corresponded with Mr. Wallace on these questions—see for instance the "Life and Letters," III., page 89; but, owing to various interruptions, serious work on the subject did not begin until 1869. The following quotations show the line of work undertaken early in 1868.
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.
I go on working at sexual selection, and, though never idle, I am able to do so little work each day that I make very slow progress. I knew from Azara about the young of the tapir being striped, and about young deer being spotted (432/2. Fritz Muller's views are discussed in the "Descent of Man," Edition II., Volume II., page 305.); I have often reflected on this subject, and know not what to conclude about the loss of the stripes and spots. From the geographical distribution of the striped and unstriped species of Equus there seems to be something very mysterious about the loss of stripes; and I cannot persuade myself that the common ass has lost its stripes owing to being rendered more conspicuous from having stripes and thus exposed to danger.
LETTER 433. TO J. JENNER WEIR.
(433/1. Mr. John Jenner Weir, to whom the following letters are addressed, is frequently quoted in the "Descent of Man" as having supplied Mr. Darwin with information on a variety of subjects.)
Down, February 27th {1868}.
I must thank you for your paper on apterous lepidoptera (433/2. Published by the West Kent Natural History, Microscopical and Photographic Society, Greenwich, 1867. Mr. Weir's paper seems chiefly to have interested Mr. Darwin as affording a good case of gradation in the degree of degradation of the wings in various species.), which has interested me exceedingly, and likewise for the very honourable mention which you make of my name. It is almost a pity that your paper was not published in some Journal in which it would have had a wider distribution. It contained much that was new to me. I think the part about the relation of the wings and spiracles and tracheae might have been made a little clearer. Incidentally, you have done me a good service by reminding me of the rudimentary spurs on the legs of the partridge, for I am now writing on what I have called sexual selection. I believe that I am not mistaken in thinking that you have attended much to birds in confinement, as well as to insects. If you could call to mind any facts bearing on this subject, with birds, insects, or any animals—such as the selection by a female of any particular male—or conversely of a particular female by a male, or on the rivalry between males, or on the allurement of the females by the males, or any such facts, I should be most grateful for the information, if you would have the kindness to communicate it.