pplied with blood-vessels, it grew in one case in a spiral direction to a
length of six inches, and in another case forward, like a horn, so that the
bird could not touch the ground with its beak. But whether Mr. Herbert
Spencer's view of the exudation of nutritive matter due to increased
movement and pressure, will fully account for the augmented size of bones,
ligaments, and especially of internal glands and nerves, seems doubtful.
According to the interesting observations of M. Sedillot,[729] when a
portion of one bone of the leg or fore-arm of an animal is removed and is
not replaced by growth, the associated bone enlarges till it attains a bulk
equal to that of the two bones, of which it has to perform the functions.
This is best exhibited in dogs in which the tibia has been removed; the
companion bone, which is naturally almost filiform and not one-fifth the
size of the other, soon acquires a size equal to or greater than the tibia.
Now, it is at first difficult to believe that increased weight acting on a
straight bone could, by alternately increased and diminished pressure,
cause nutritive matter to exude from the vessels which permeate the
periosteum. Nevertheless, the observations adduced by Mr. Spencer,[730] on
the strengthening of the bowed bones of rickety children, along their
concave sides, leads to the belief that this is possible.

Mr. H. Spencer has also shown that the ascent of the sap in trees is aided
by the rocking movement caused by the wind; and the sap strengthens the
trunk "in proportion to the stress to be borne; since the more severe and
the more repeated the strains, the greater must be the exudation from the
vessels into the surrounding tissue, and the greater the thickening of this
tissue by secondary deposits."[731] But woody trunks may be formed of hard
tissue without their having been subjected to any movement, as we see with
ivy closely attached to old walls. In all these cases, it is very difficult
to disentangle the effects of long-continued selection from those
consequent on the increased action or movement of the part. Mr. H.
Spencer[732] acknowledges this difficulty, and gives as an instance the
spines {297} or thorns of trees, and the shells of nuts. Here we have
extremely hard woody tissue without the possibility of any movement to
cause exudation, and without, as far as we can see, any other directly
exciting cause; and as the hardness of these parts is of manifest service
to the