required by our theory, few will be inclined
to admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the
manner required by the theory, for they have changed slowly and in
a graduated manner. 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. There is much difficulty in ascertaining how
largely our domestic productions have been modified; but we may safely
infer that the amount has been large, and that modifications can be
inherited for long periods. As long as the conditions of life remain the
same, we have reason to believe that a modification, which has already
been inherited for many generations, may continue to be inherited for
an almost infinite number of generations. On the other hand we have
evidence that variability, when it has once come into play, does not
cease under domestication for a very long period; nor do we know that
it ever ceases, for new varieties are still occasionally produced by our
oldest dom