How, it may be asked, in the 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