of
comparison be taken, neither the leg-bones nor the scapulae have
increased in length proportionally with the increased dimensions of the
rest of the skeleton. The skull has become in a marked manner narrower,
and, from the measurements of its capacity formerly given, we may
conclude, that this narrowness results from the decreased size of the
brain, consequent on the mentally inactive life led by these
closely-confined animals.

We have seen in the eighth chapter that silk-moths, which have been
kept during many centuries closely confined, emerge from their cocoons
with their wings distorted, incapable of flight, often greatly reduced
in size, or even, according to Quatrefages, quite rudimentary. This
condition of the wings may be largely owing to the same kind of
monstrosity which often affects wild Lepidoptera when artificially
reared from the cocoon; or it may {299} be in part due to an inherent
tendency, which is common to the females of many Bombycidae, to have
their wings in a more or less rudimentary state; but part of the effect
may probably be attributed to long-continued disuse.

From the foregoing facts there can be no doubt that certain parts of the
skeleton in our anciently domesticated animals, have been modified in
length and weight by the effects of decreased or increased use; but they
have not been modified, as shown in the earlier chapters, in shape or
structure. We must, however, be cautious in extending this latter
conclusion to animals living a free life; for these will occasionally be
exposed during successive generations to the severest competition. With
wild animals it would be an advantage in the struggle for life that every
superfluous and useless detail of structure should be removed or absorbed;
and thus the reduced bones might ultimately become changed in structure.
With highly-fed domesticated animals, on the other hand, there is no
economy of growth; nor any tendency to the elimination of trifling and
superfluous details of structure.

Turning now to more general observations, Nathusius has shown that, with
the improved races of the pig, the shortened legs and snout, the form of
the articular condyles of the occiput, and the position of the jaws with
the upper canine teeth projecting in a most anomalous manner in front of
the lower canines, may be attributed to these parts not having been fully
exercised. For the high