annot be supposed that the peculiar speckled
appearance of the upper side of the sole, so like the sandy bed of the
sea, or the power in some species, as recently shown by Pouchet, of
changing their colour in accordance with the surrounding surface, or the
presence of bony tubercles on the upper side of the turbot, are due to
the action of the light. Here natural selection has probably come into
play, as well as in adapting the general shape of the body of these
fishes, and many other peculiarities, to their habits of life. We should
keep in mind, as I have before insisted, that the inherited effects
of the increased use of parts, and perhaps of their disuse, will be
strengthened by natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those individuals
which inherit in the highest degree the effects of the increased and
beneficial use of any part. How much to attribute in each particular
case to the effects of use, and how much to natural selection, it seems
impossible to decide.

I may give another instance of a structure which apparently owes its
origin exclusively to use or habit. The extremity of the tail in
some American monkeys has been converted into a wonderfully perfect
prehensile organ, and serves as a fifth hand. A reviewer, who agrees
with Mr. Mivart in every detail, remarks on this structure: "It is
impossible to believe that in any number of ages the first slight
incipient tendency to grasp could preserve the lives of the individuals
possessing it, or favour their chance of having and of rearing
offspring." But there is no necessity for any such belief. Habit, and
this almost implies that some benefit great or small is thus derived,
would in all probability suffice for the work. Brehm saw the young of
an African monkey (Cercopithecus) clinging to the under surface of their
mother by their hands, and at the same time they hooked their little
tails round that of their mother. Professor Henslow kept in confinement
some harvest mice (Mus messorius) which do not possess a structurally
prehensive tail; but he frequently observed that they curled their
tails round the branches of a bush placed in the cage, and thus aided
themselves in climbing. I have received an analogous account from Dr.
Gunther, who has seen a mouse thus suspend itself. If the harvest mouse
had been more strictly arboreal, it would perhaps have had its tail
rendered structurally prehensile, a