animal or plant, or towards some dead or
inorganic matter. But as, according to Mr. Darwin's theory, there is a
constant tendency to indefinite variation, and as the minute incipient
variations will be in _all directions_, they must tend to neutralize each
other, and at first to form such unstable modifications that it is
difficult, if not impossible, to see how such indefinite oscillations of
infinitesimal beginnings can ever build up a sufficiently appreciable
resemblance to a leaf, bamboo, or other object, for "Natural Selection" to
seize upon and perpetuate. This difficulty is augmented when we consider--a
point to be dwelt upon hereafter--how necessary it is that many individuals
should be similarly modified simultaneously. This has been insisted on in
an able article in the _North British Review_ for June 1867, p. 286, and
the consideration of the article has occasioned Mr. Darwin to make an
important modification in his views.[28]

In these cases of mimicry it seems difficult indeed to imagine a reason why
variations tending in an _infinitesimal degree_ in any special direction
should be preserved. All variations would be preserved which tended to
obscure the perception of an animal by its enemies, whatever direction
those variations might take, and the common preservation of conflicting
tendencies would greatly favour their mutual neutralization and
obliteration if we may rely on the many cases recently brought forward by
Mr. Darwin with regard to domestic animals.

[Illustration: THE WALKING-LEAF INSECT.]

Mr. Darwin explains the imitation of some species by others more or less
nearly allied to it, by the common origin of both the mimic and the
mimicked species, and the consequent possession by both (according to the
theory of "Pangenesis") of gemmules tending to reproduce ancestral
characters, which characters the mimic must be assumed first to have {35}
lost and then to have recovered. Mr. Darwin says,[29] "Varieties of one
species frequently mimic distinct species, a fact in perfect harmony with
the foregoing cases, and explicable _only on the theory of descent_." But
this at the best is but a partial and very incomplete explanation. It is
one, moreover, which Mr. Wallace does not accept.[30] It is very
incomplete, because it has no bearing on some of the most striking cases,
and of course Mr. Darwin does not pretend that it has. We should have to go
back far indeed to reach the common ancestor of the mim