"Origin" (6th edition) London,
1872, pages 181, 182; see also page 66.), never entirely agreed with
Wallace's estimate of its importance. Thus the following extract from a
letter to Sir Joseph Hooker, May 21, 1868, refers to Wallace: "I find
I must (and I always distrust myself when I differ from him) separate
rather widely from him all about birds' nests and protection; he is
riding that hobby to death." ("More Letters", I. page 304.) It is clear
from the account given in "The Descent of Man", (London, 1874, pages
452-458. See also "Life and Letters", III. pages 123-125, and "More
Letters", II. pages 59-63, 72-74, 76-78, 84-90, 92, 93.), that the
divergence was due to the fact that Darwin ascribed more importance
to Sexual Selection than did Wallace, and Wallace more importance to
Protective Resemblance than Darwin. Thus Darwin wrote to Wallace,
Oct. 12 and 13, 1867: "By the way, I cannot but think that you push
protection too far in some cases, as with the stripes on the tiger."
("More Letters", I. page 283.) Here too Darwin was preferring the
explanation offered by Sexual Selection ("Descent of Man" (2nd edition)
1874, pages 545, 546.), a preference which, considering the relation of
the colouring of the lion and tiger to their respective environments,
few naturalists will be found to share. It is also shown that Darwin
contemplated the possibility of cryptic colours such as those of
Patagonian animals being due to sexual selection influenced by the
aspect of surrounding nature.

Nearly a year later Darwin in his letter of May 5, 1868?, expressed
his agreement with Wallace's views: "Expect that I should put sexual
selection as an equal, or perhaps as even a more important agent in
giving colour than Natural Selection for protection." ("More Letters",
II. pages 77, 78.) The conclusion expressed in the above quoted passage
is opposed by the extraordinary development of Protective Resemblance in
the immature stages of animals, especially insects.

It must not be supposed, however, that Darwin ascribed an unimportant
role to Cryptic Resemblances, and as observations accumulated he came to
recognise their efficiency in fresh groups of the animal kingdom. Thus
he wrote to Wallace, May 5, 1867: "Haeckel has recently well shown that
the transparency and absence of colour in the lower oceanic animals,
belonging to the most different classes, may be well accounted for on
the principle of protection." ("More Letters", II. page