ering that the proportion of living and known forms to the extinct
and unknown 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 but somewhat exaggerated 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 commonly 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 {195} 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 its 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,