In most cases only general reasons, but
in some few cases special reasons, can be assigned. Thus to adapt a
species to new habits of life, many co-ordinated modifications are almost
indispensable, and it may often have happened that the requisite parts did
not vary in the right manner or to the right degree. Many species must
have been prevented from increasing in numbers through destructive
agencies, which stood in no relation to certain structures, which we
imagine would have been gained through natural selection from appearing to
us advantageous to the species. In this case, as the struggle for life did
not depend on such structures, they could not have been acquired through
natural selection. In many cases complex and long-enduring conditions,
often of a peculiar nature, are necessary for the development of a
structure; and the requisite conditions may seldom have concurred. The
belief that any given structure, which we think, often erroneously, would
have been beneficial to a species, would have been gained under all
circumstances through natural selection, is opposed to what we can
understand of its manner of action. Mr. Mivart does not deny that natural
selection has effected something; but he considers it as "demonstrably
insufficient" to account for the phenomena which I explain by its agency.
His chief arguments have now been considered, and the others will hereafter
be considered.
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