On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit
saltum." This canon, if we look only to the present inhabitants of the
world, is not strictly correct, but if we include all those of past
times, it must by my theory be strictly true.
It is generally acknowledged that all organic beings have been formed
on two great laws--Unity of Type, and the Conditions of Existence. By
unity of type is meant that fundamental agreement in structure, which
we see in organic beings of the same class, and which is quite
independent of their habits of life. On my theory, unity of type is
explained by unity of descent. The expression of conditions of
existence, so often insisted on by the illustrious Cuvier, is fully
embraced by the principle of natural selection. For natural selection
acts by either now adapting the varying parts of each being to its
organic and inorganic conditions of life; or by having adapted them
during long-past periods of time: the adaptations being aided in some
cases by use and disuse, being slightly affected by the direct action
of the external conditions of life, and being in all cases subjected
to the several laws of growth. Hence, in fact, the law of the
Conditions of Existence is the higher law; as it includes, through the
inheritance of former adaptations, that of Unity of Type.
CHAPTER 7. INSTINCT.
Instincts comparable with habits, but different in their origin.
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