ell 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.

In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to
spontaneous variability. But it is impossible to attribute to this cause
the innumerable structures which are so well adapted to the habits
of life of each species. I can no more believe in this than that
the well-adapted form of a race-horse or greyhound, which before the
principle of selection by man was well understood, excited so much
surprise in the minds of the older naturalists, can thus be explained.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is
hardly necessary to observe that even in the higher and best-known
animals many structures exist, which are so highly developed that no one
doubts that they are of importance, yet their use has not been, or has
only recently been, ascertained. As Bronn gives the length of the ears
and tail in the several species of mice as instances, though trifling
ones, of differences in structure which can be of no special use, I may
mention that, according to Dr. Schobl, the external ears of the common
mouse are supplied in an extraordinary manner with nerves, so that
they no doubt serve as tactile organs; hence the length of the ears can
hardly be quite unimportant. We shall, also, presently see that the tail
is a highly useful prehensile organ to some of the species; and its use
would be much influence by its length.

With respect to plants, to which on account of Nageli's essay I shall
confine myself in the following remarks, it will be admitted that the
flowers of the orchids present a multitude of curious structures,
which a few years ago would have been considered as mere morphological
differences without any special function; but they are now known to be
of the highest importance for the fertilisation of the species through
the aid of insects, and have probably been gained through natural
selection. No one until lately would have imagined that in dimorphic and
trimorphic plants the different lengths of the stamens and pistils, and
their arrangement, could have been of any service, but now we know this
to be the case.

In certain whole groups of