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.
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