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On the origin of species;The Origin of Species by means of Natural Selection, 6th Edition by Charles Darwin
page 249 of 685 (36%)
perfect development. He specifies the arrangement of the cells in the
tissues, and of the leaves on the axis, as cases in which natural selection
could not have acted. To these may be added the numerical divisions in the
parts of the flower, the position of the ovules, the shape of the seed,
when not of any use for dissemination, etc.

There is much force in the above objection. Nevertheless, we ought, in the
first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In the
second place, it should always be borne in mind that when one part is
modified, so will be other parts, through certain dimly seen causes, such
as an increased or diminished flow of nutriment to a part, mutual pressure,
an early developed part affecting one subsequently developed, and so forth
--as well as through other causes which lead to the many mysterious cases
of correlation, which we do not in the least understand. These agencies
may be all grouped together, for the sake of brevity, under the expression
of the laws of growth. In the third place, we have to allow for the direct
and definite action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the conditions apparently
plays a quite subordinate part. Bud-variations, such as the appearance of
a moss-rose on a common rose, or of a nectarine on a peach-tree, offer good
instances of spontaneous variations; but even in these cases, if we bear in
mind the power of a minute drop of poison in producing complex galls, we
ought not to feel too sure that the above variations are not the effect of
some local change in the nature of the sap, due to some change in the
conditions. There must be some efficient cause for each slight individual
difference, as well as for more strongly marked variations which
occasionally arise; and if the unknown cause were to act persistently, it
is almost certain that all the individuals of the species would be
similarly modified.
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