the very same invention, so natural selection, working for the good of
each being and taking advantage of analogous variations, has sometimes
modified in very nearly the same manner two parts in two organic
beings, which owe but little of their structure in common to
inheritance from the same ancestor.
Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering 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 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
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