We clearly see this in the fossil remains from consecutive
formations invariably being much more closely related to each other than
are the fossils from widely separated formations.
Such is the sum of the several chief objections and difficulties which may
justly be urged against the theory; and I have now briefly recapitulated
the answers and explanations which, as far as I can see, may be given. I
have felt these difficulties far too heavily during many years to doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor do
we know how ignorant we are. We do not know all the possible transitional
gradations between the simplest and the most perfect organs; it cannot be
pretended that we know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect is the Geological Record.
Serious as these several objections are, in my judgment they are by no
means sufficient to overthrow the theory of descent with subsequent
modification.
Now let us turn to the other side of the argument. Under domestication we
see much variability, caused, or at least excited, by changed conditions of
life; but often in so obscure a manner, that we are tempted to consider the
variations as spontaneous. Variability is governed by many complex laws,
by correlated growth, compensation, the increased use and disuse of parts,
and the definite action of the surrounding conditions.
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