being staggered. Such
papers will make many more converts among naturalists than long-winded
books such as I shall write if I have strength. I have been particularly
struck with your remarks on dimorphism; but I cannot quite understand
one point (page 22), (189/3. The passage referred to in this letter as
needing further explanation is the following: "The last six cases of
mimicry are especially instructive, because they seem to indicate one of
the processes by which dimorphic forms have been produced. When, as in
these cases, one sex differs much from the other, and varies greatly
itself, it may happen that individual variations will occasionally
occur, having a distant resemblance to groups which are the objects
of mimicry, and which it is therefore advantageous to resemble. Such
a variety will have a better chance of preservation; the individuals
possessing it will be multiplied; and their accidental likeness to the
favoured group will be rendered permanent by hereditary transmission,
and each successive variation which increases the resemblance being
preserved, and all variations departing from the favoured type having
less chance of preservation, there will in time result those singular
cases of two or more isolated and fixed forms bound together by that
intimate relationship which constitutes them the sexes of a single
species. The reason why the females are more subject to this kind of
modification than the males is, probably, that their slower flight,
when laden with eggs, and their exposure to attack while in the act of
depositing their eggs upon leaves, render it especially advantageous
for them to have some additional protection. This they at once obtain
by acquiring a resemblance to other species which, from whatever cause,
enjoy a comparative immunity from persecution." Mr. Wallace has been
good enough to give us the following note on the above passage: "The
above quotation deals solely with the question of how certain females
of the polymorphic species (Papilio Memnon, P. Pammon, and others) have
been so modified as to mimic species of a quite distinct section of
the genus; but it does not attempt to explain why or how the other very
variable types of female arose, and this was Darwin's difficulty. As
the letter I wrote in reply is lost, and as it is rather difficult to
explain the matter clearly without reference to the coloured figures,
I must go into some little detail, and give now what was probably the