ainst the right variation or combination of variations occurring just
when required; and further, that no variation can be perpetuated that is
not accompanied by several concomitant variations of dependent
parts--greater length of a wing in a bird, for example, would be of
little use if unaccompanied by increased volume or contractility of the
muscles which move it. This objection seemed a very strong one so long
as it was supposed that variations occurred singly and at considerable
intervals; but it ceases to have any weight now we know that they occur
simultaneously in various parts of the organism, and also in a large
proportion of the individuals which make up the species. A considerable
number of individuals will, therefore, every year possess the required
combination of characters; and it may also be considered probable that
when the two characters are such that they always _act_ together, there
will be such a correlation between them that they will frequently _vary_
together. But there is another consideration that seems to show that
this coincident variation is not essential. All animals in a state of
nature are kept, by the constant struggle for existence and the survival
of the fittest, in such a state of perfect health and usually
superabundant vigour, that in all ordinary circumstances they possess a
surplus power in every important organ--a surplus only drawn upon in
cases of the direst necessity when their very existence is at stake. It
follows, therefore, that _any_ additional power given to one of the
component parts of an organ must be useful--an increase, for example,
either in the wing muscles or in the form or length of the wing might
give _some_ increased powers of flight; and thus alternate
variations--in one generation in the muscles, in another generation in
the wing itself--might be as effective in permanently improving the
powers of flight as coincident variations at longer intervals. On either
supposition, however, this objection appears to have little weight if we
take into consideration the large amount of coincident variability that
has been shown to exist.

_The Beginnings of Important Organs._

We now come to an objection which has perhaps been more frequently urged
than any other, and which Darwin himself felt to have much weight--the
first beginnings of important organs, such, for example, as wings, eyes,
mammary glands, and numerous other structures. It is urged, that it is
almost i