and therefore cannot have been influenced through
natural selection. Bronn adduces the length of the ears and tails in the
different species of hares and mice--the complex folds of enamel in the
teeth of many animals, and a multitude of analogous cases. With respect
to plants, this subject has been discussed by Nageli in an admirable
essay. He admits that natural selection has effected much, but he
insists that the families of plants differ chiefly from each other in
morphological characters, which appear to be quite unimportant for the
welfare of the species. He consequently believes in an innate tendency
towards progressive and more 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 w