gree than other parts;
for variation is a long-continued and slow process, and natural selection
will in such cases not as yet have had time to overcome the tendency to
further variability and to reversion to a less modified state. 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 {170} 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.

* * * * *

{171}

CHAPTER VI.

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.

Th