. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants--which on my view must be a very slow process, requiring
a long lapse of time--in this case, natural selection may readily
have succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar
influences will naturally tend to present analogous variations, and
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 in the offspring
from their parents--and a cause for each must exist--it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face
of this earth are enabled to struggle with each other, and the best
adapted to survive.

6. DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification. Transitions.
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.
Means 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 having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so grave
that to this day I can never reflect on them without being 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 my theory.

These difficulties and objections may be classed under the following
heads:--

Firstly, why, if species have descended from other species by insensibly
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,