of birds, are rare and exceptional. On this head, Mr. Mivart remarks, "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."

But in all the foregoing cases the insects in their original state no doubt
presented some rude and accidental resemblance to an object commonly found
in the stations frequented by them. Nor is this at all improbable,
considering the almost infinite number of surrounding objects and the
diversity in form and colour of the hosts of insects which exist. As some
rude resemblance is necessary for the first start, we can understand how it
is that the larger and higher animals do not (with the exception, as far as
I know, of one fish) resemble for the sake of protection special objects,
but only the surface which commonly surrounds them, and this chiefly in
colour. Assuming that an insect originally happened to resemble in some
degree a dead twig or a decayed leaf, and that it varied slightly in many
ways, then all the variations which rendered the insect at all more like
any such object, and thus favoured its escape, would be preserved, while
other variations would be neglected and ultimately lost; or, if they
rendered the insect at all less like the imitated object, they would be
eliminated. There would indeed be force in Mr. Mivart's objection, if we
were to attempt to account for the above resemblances, independently of
natural selection, through mere fluctuating variability; but as the case
stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the