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DARWINISM
CHAP.
organs of a species of earthworm (Perionyx excavatus). The normal characters of this species are—
- Setæ forming a complete row round each segment.
- Two pairs of spermathecæ—spherical pouches without diverticulæ—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 spermathecæ 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.
When we consider the enormous number of earthworms and the comparatively very small number of individuals examined, we may be sure, not only that such variations as these occur with considerable frequency, but also that still more extraordinary deviations from the normal structure may often exist.
The next example is taken from Mr. Darwin's unpublished MSS.
