en shows, that, both the general amount of the variation and the
independent variability of the several members of the body, accord
completely with the variations so common in the class of birds; while
their amount and their independence of each other are even greater than
usual.

_Variation in the Internal Organs of Animals._

In case it should be objected that the cases of variation hitherto
adduced are in the external parts only, and that there is no proof that
the internal organs vary in the same manner, it will be advisable to
show that such varieties also occur. It is, however, impossible to
adduce the same amount of evidence in this class of variation, because
the great labour of dissecting large numbers of specimens of the same
species is rarely undertaken, and we have to trust to the chance
observations of anatomists recorded in their regular course of study.

It must, however, be noted that a very large proportion of the
variations already recorded in the external parts of animals necessarily
imply corresponding internal variations. When feet and legs vary in
size, it is because the bones vary; when the head, body, limbs, and tail
change their proportions, the bony skeleton must also change; and even
when the wing or tail feathers of birds become longer or more numerous,
there is sure to be a corresponding change in the bones which support
and the muscles which move them. I will, however, give a few cases of
variations which have been directly observed.

[Illustration: FIG. 13.--Sciurus carolinensis. 32 specimens. Florida.]

Mr. Frank E. Beddard has kindly communicated to me some remarkable
variations he has observed in the internal organs of a species of
earthworm (Perionyx excavatus). The normal characters of this species
are--

Setae forming a complete row round each segment.

Two pairs of spermathecae--spherical pouches without
diverticulae--in segments 8 and 9.

Two pairs of testes in segments 11 and 12.

Ovaries, a single pair in segment 13.

Oviducts open by a common pore in the middle of segment 14.

Vasa deferentia open separately in segment 18, each furnished at
its termination with a large prostate gland.

Between two and three hundred specimens were examined, and among them
thirteen specimens exhibited the following marked variations:--

(1) The number of the spermathecae varied from two to three or
four pairs, their position also varyi