Yet the deviation {34} must, as the event has shown, in each case be in some definite direction, whether it be towards some other 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.