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.
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