foregoing and in innumerable other
instances, can we understand the graduated scale of complexity and the
multifarious means for gaining the same end. The answer no doubt is, as
already remarked, that when two forms vary, which already differ from
each other in some slight degree, the variability will not be of the
same exact nature, and consequently the results obtained through natural
selection for the same general purpose will not be the same. We should
also bear in mind that every highly developed organism has passed
through many changes; and that each modified structure tends to be
inherited, so that each modification will not readily be quite lost,
but may be again and again further altered. Hence, the structure of each
part of each species, for whatever purpose it may serve, is the sum of
many inherited changes, through which the species has passed during its
successive adaptations to changed habits and conditions of life.

Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs could have arrived at their
present state; yet, considering how small the proportion of living and
known forms is to the extinct and unknown, I have been astonished how
rarely an organ can be named, towards which no transitional grade is
known to lead. It is certainly true, that new organs appearing as if
created for some special purpose rarely or never appear in any being; as
indeed is 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 there be so much
variety and so little real novelty? Why should all the parts and organs
of many independent beings, each supposed to have been separately
created for its own proper place in nature, be so commonly linked
together by graduated steps? Why should not Nature take a sudden leap
from structure to structure? On the theory of natural selection, we can
clearly understand why she should not; for natural selection acts only
by taking advantage of slight successive variations; she can never take
a great and sudden leap, but must advance by the short and sure, though
slow steps.

ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION.

As natural selection acts by