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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
explanation I gave at the time. The male of Papilio Memnon is a large
black butterfly with the nervures towards the margins of the wings
bordered with bluish gray dots. It is a forest insect, and the very
dark colour renders it conspicuous; but it is a strong flier, and thus
survives. To the female, however, this conspicuous mass of colour
would be dangerous, owing to her slower flight, and the necessity for
continually resting while depositing her eggs on the leaves of the
food-plant of the larva. She has accordingly acquired lighter and more
varied tints. The marginal gray-dotted stripes of the male have become
of a brownish ash and much wider on the fore wings, while the margin
of the hind wings is yellowish, with a more defined spot near the
anal angle. This is the form most nearly like the male, but it is
comparatively rare, the more common being much lighter in colour, the
bluish gray of the hind wings being often entirely replaced by a broad
band of yellowish white. The anal angle is orange-yellow, and there is
a bright red spot at the base of the fore wings. Between these two
extremes there is every possible variation. Now, it is quite certain
that this varying mixture of brown, black, white, yellow, and red is far
less conspicuous amid the ever-changing hues of the forest with their
glints of sunshine everywhere penetrating so as to form strong contrasts
and patches of light and shade. Hence ALL the females--one at one time
and one at another--get SOME protection, and that is sufficient to
enable them to live long enough to lay their eggs, when their work is
finished. Still, under bad conditions they only just managed to
survive, and as the colouring of some of these varying females very
much resembled that of the protected butterflies of the P. coon
group (perhaps at a time when the tails of the latter were not fully
developed) any rudiments of a prolongation of the wing into a tail added
to the protective resemblance, and was therefore preserved. The woodcuts
of some of these forms in my "Malay Archipelago" (i., page 200) will
enable those who have this book at hand better to understand the
foregoing explanation."), and should be grateful for an explanation, for
I want fully to understand you. How can one female form be selected and
the intermediate forms die out, without also the other extreme form also
dying out from not having the advantages of the first selected form?
for, as I understand, both female forms occur on the same island. I
quite agree with your distinction between dimorphic forms and varieties;
but I doubt whether your criterion of dimorphic forms not producing
intermediate offspring will suffice, for I know of a good many varieties
which must be so called that will not blend or intermix, but produce
offspring quite like either parent.

I have been particularly struck with your remarks on geographical
distribution in Celebes. It is impossible that anything could be better
put, and would give a cold shudder to the immutable naturalists.

And now I am going to ask a question which you will not like. How does
your journal get on? It will be a shame if you do not popularise your
researches.

LETTER 190. A.R. WALLACE TO CHARLES DARWIN. Hurstpierpoint, Sussex, July
2nd, 1866.

I have been so repeatedly struck by the utter inability of numbers
of intelligent persons to see clearly, or at all, the self-acting and
necessary effects of Natural Selection, that I am led to conclude that
the term itself, and your mode of illustrating it, however clear and
beautiful to many of us, are yet not the best adapted to impress it
on the general naturalist public. The two last cases of the
misunderstanding are: (1) the article on "Darwin and his Teachings" in
the last "Quarterly Journal of Science," which, though very well written
and on the whole appreciative, yet concludes with a charge of something
like blindness, in your not seeing that Natural Selection requires the
constant watching of an intelligent "chooser," like man's selection to
which you so often compare it; and (2) in Janet's recent work on the
"Materialism of the Present Day," reviewed in last Saturday's "Reader,"
by an extract from which I see that he considers your weak point to be
that you do not see that "thought and direction are essential to the
action of Natural Selection." The same objection has been made a score
of times by your chief opponents, and I have heard it as often stated
myself in conversation. Now, I think this arises almost entirely from
your choice of the term "Natural Selection" and so constantly comparing
it in its effec
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