ows which should
first yield much milk, and afterwards be capable of {344} fattening well.
With fowls which have large topknots and beards the comb and wattles are
generally much reduced in size. Perhaps the entire absence of the oil-gland
in fantail pigeons may be connected with the great development of their
tails.

* * * * *

_Mechanical Pressure as a Cause of Modifications._--In some few cases there
is reason to believe that mere mechanical pressure has affected certain
structures. Every one knows that savages alter the shape of their infants'
skulls by pressure at an early age; but there is no reason to believe that
the result is ever inherited. Nevertheless Vrolik and Weber[852] maintain
that the shape of the human head is influenced by the shape of the mother's
pelvis. The kidneys in different birds differ much in form, and St.
Ange[853] believes that this is determined by the form of the pelvis, which
again, no doubt, stands in close relation with their various habits of
locomotion. In snakes, the viscera are curiously displaced, in comparison
with their position in other vertebrates; and this has been attributed by
some authors to the elongation of their bodies; but here, as in so many
previous cases, it is impossible to disentangle any direct result of this
kind from that consequent on natural selection. Godron has argued[854] that
the normal abortion of the spur on the inner side of the flower in
Corydalis, is caused by the buds being closely pressed at a very early
period of growth, whilst under ground, against each other and against the
stem. Some botanists believe that the singular difference in the shape both
of the seed and corolla, in the interior and exterior florets in certain
compositous and umbelliferous plants, is due to the pressure to which the
inner florets are subjected; but this conclusion is doubtful.

The facts just given do not relate to domesticated productions, and
therefore do not strictly concern us. But here is a more appropriate case:
H. Mueller[855] has shown that in {345} short-faced races of the dog some
of the molar teeth are placed in a slightly different position from that
which they occupy in other dogs, especially in those having elongated
muzzles; and as he remarks, any inherited change in the arrangement of the
teeth deserves notice, considering their classificatory importance. This
difference in position is due to the shortening of certain fa