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 past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being affected
by the direct action of external conditions of life, and subjected in all
cases to the several laws of growth and variation. Hence, in fact, the law
of the Conditions of Existence is the higher law; as it includes, through
the inheritance of former variations and adaptations, that of Unity of
Type.
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
Longevity -- Modifications not necessarily simultaneous -- Modifications
apparently of no direct service -- Progressive development -- Characters of
small functional importance, the most constant -- Supposed incompetence of
natural selection to account for the incipient stages of useful structures
-- Causes which interfere with the acquisition through natural selection of
useful structures -- Gradations of structure with changed functions --
Widely different organs in members of the same class, developed from one
and the same source -- Reasons for disbelieving in great and abrupt
modifications.
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