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 varying. (2) There were occasionally two pairs of ovaries, each with its own oviduct; the external apertures of these varied in position, being upon segments 13 and 14, 14 and 15, or 15 and 16. Occasionally when there was only the normal single oviduct pore present it varied in position, once occurring on the 10th, and once on the 11th segment. (3) The male generative pores varied in position from segments 14 to 20. In one instance there were two pairs instead of the normal single pair, and in this case each of the four apertures had its own prostate gland. Mr. Beddard remarks that all, or nearly all, the above variations are found _normally_ in other genera and species.