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.
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.