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