be preserved till
we reached the perfect eye of birds and mammals. Even this, as we know,
is not absolutely, but only relatively, perfect. Neither the chromatic
nor the spherical aberration is absolutely corrected; while long-and
short-sightedness, and the various diseases and imperfections to which
the eye is liable, may be looked upon as relics of the imperfect
condition from which the eye has been raised by variation and natural
selection.

These few examples of difficulties as to the origin of remarkable or
complex organs must suffice here; but the reader who wishes further
information on the matter may study carefully the whole of the sixth
and seventh chapters of the last edition of _The Origin of Species_, in
which these and many other cases are discussed in considerable detail.

_Useless or non-adaptive Characters._

Many naturalists seem to be of opinion that a considerable number of the
characters which distinguish species are of no service whatever to their
possessors, and therefore cannot have been produced or increased by
natural selection. Professors Bronn and Broca have urged this objection
on the continent. In America, Dr. Cope, the well-known palaeontologist,
has long since put forth the same objection, declaring that non-adaptive
characters are as numerous as those which are adaptive; but he differs
completely from most who hold the same general opinion in considering
that they occur chiefly "in the characters of the classes, orders,
families, and other higher groups;" and the objection, therefore, is
quite distinct from that in which it is urged that "specific characters"
are mostly useless. More recently, Professor G.J. Romanes has urged this
difficulty in his paper on "Physiological Selection" (_Journ. Linn.
Soc._, vol. xix. pp. 338, 344). He says that the characters "which serve
to distinguish allied species are frequently, if not usually, of a kind
with which natural selection can have had nothing to do," being without
any utilitarian significance. Again he speaks of "the enormous number,"
and further on of "the innumerable multitude" of specific peculiarities
which are useless; and he finally declares that the question needs no
further arguing, "because in the later editions of his works Mr. Darwin
freely acknowledges that a large proportion of specific distinctions
must be conceded to be useless to the species presenting them."

I have looked in vain in Mr. Darwin's works to find any such