ercome the tendency to further variability and to
reversion to a less modified state. But when a species with an
extraordinarily developed organ has become the parent of many modified
descendants--which on our view must be a very slow process, requiring a
long lapse of time--in this case, natural selection has succeeded in
giving a fixed character to the organ, in however extraordinary a manner
it may have been developed. Species inheriting nearly the same
constitution from a common parent, and exposed to similar influences,
naturally tend to present analogous variations, or these same species
may occasionally revert to some of the characters of their ancient
progenitors. Although new and important modifications may not arise from
reversion and analogous variation, such modifications will add to the
beautiful and harmonious diversity of nature.

Whatever the cause may be of each slight difference between the
offspring and their parents--and a cause for each must exist--we have
reason to believe that it is the steady accumulation of beneficial
differences which has given rise to all the more important modifications
of structure in relation to the habits of each species.

CHAPTER VI. DIFFICULTIES OF THE THEORY.

Difficulties of the theory of descent with modification--Absence
or rarity of transitional varieties--Transitions in habits of
life--Diversified habits in the same species--Species with habits
widely different from those of their allies--Organs of extreme
perfection--Modes of transition--Cases of difficulty--Natura non facit
saltum--Organs of small importance--Organs not in all cases absolutely
perfect--The law of Unity of Type and of the Conditions of Existence
embraced by the theory of Natural Selection.

Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that
to this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the theory.

These difficulties and objections may be classed under the following
heads: First, why, if species have descended from other species by fine
gradations, do we not everywhere see innumerable transitional forms? Why
is not all nature in confusion, instead of the species being, as we see
them, well defined?

Secondly, is it possible that a