is very small, I have been astonished how rarely an organ
can be named, towards which no transitional grade is known to lead.
The truth of this remark is indeed shown by that old canon in natural
history of "Natura non facit saltum." We meet with this admission in the
writings of almost every experienced naturalist; or, as Milne Edwards
has well expressed it, nature is prodigal in variety, but niggard in
innovation. Why, on the theory of Creation, should this be so? Why
should all the parts and organs of many independent beings, each
supposed to have been separately created for its proper place in nature,
be so invariably linked together by graduated steps? Why should not
Nature have taken a leap from structure to structure? On the theory of
natural selection, we can clearly understand why she should not; for
natural selection can act only by taking advantage of slight successive
variations; she can never take a leap, but must advance by the shortest
and slowest steps.

ORGANS OF LITTLE APPARENT IMPORTANCE.

As natural selection acts by life and death,--by the preservation of
individuals with any favourable variation, and by the destruction of
those with any unfavourable deviation of structure,--I have sometimes
felt much difficulty in understanding the origin of simple parts, of
which the importance does not seem sufficient to cause the preservation
of successively varying individuals. I have sometimes felt as much
difficulty, though of a very different kind, on this head, as in the
case of an organ as perfect and complex as the eye.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications would
be of importance or not. In a former chapter I have given instances of
most trifling characters, such as the down on fruit and the colour of
the flesh, which, from determining the attacks of insects or from being
correlated with constitutional differences, might assuredly be acted on
by natural selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this
could have been adapted for its present purpose by successive slight
modifications, each better and better, for so trifling an object as
driving away flies; yet we should pause before being too positive even
in this case, for we know that the distribution and existence of cattle
and other animals in South America absolutely depen