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Show 68 DARWINISM OUAP. organs of n, species of earthworm (Perionyx exmwa.tus). The norma1 characters of this ·pecies are- Setre formincr a. complete row round each segment. Two pairs ~f spermathecre-spherica.l pouches without. diverticulre-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 la.rge prostate gland. Between two and three hundred specimens were exa.minc(l, and among them thirteen specimens exhibited the following marked variations :- (1) The number of the sperma.thecro varied from two to three or four pairs, their position also varying. (2) There were occasionally two pairs of ovarie., each with its own oviduct; the external apertures of these varied in position, being upon segments 1:3 and 14, 14 and 15, or 15 and 1 G. Occasionally when there was only the normal single ovidu<'L pore present it va.ried in position, once occurring on the 1 Otb, and once on the 11th segment. (3) The male generative pores varied in position from segments 14 to 20. In one instance there were t wo pairs instead of the normal single pair, and in th is case each of the four apertures ha.d its own prostate gland. Mr. Beddard remarks tha.t all, or nearly all, tho a.hove varia.tions are found normally in other gonortt and speciei:i. When we consider the enormous number of earthworms and the comparatively very small number of individnah; c."amined, we may be sure, not only that such variations :1s 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 Darwin's unpublished MSS. Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 6!) "!n somo spodes of Shrews (~ orex) and in some flel<l-mico (ArviCola), the Hev. L. ·~ enyns (Ann. Nat. !fist., vol. vii. pp. 267, 272) foun~ tho proportwnal length of tho intestinal canal to vary considerably. He found tho arne variability in tho nun~ber of tho caudal vortohrro. In three specimens of an Arv10ola he found the gall-bladder having a. very different degre~ of development, and thoro is reason to believe it is somotnnos absent. Professor Owen has shown that this is the case w~th the gall-bla.ddor of tho giraffe." Dr. Cnsp (P1'oc. Zoot. Soc., 1862, p. 137) found tho crallbladdcr present in some specimens of Corvus supcrciliaris while absent m others; and he found it to be ab ·ent in three giraffes. which he dis~cctod. A double gall-bladder was found ~n a sheep, and m a small mammal preserved in the Huntenan Museum there arc three di tinct gall-bladders. Tho _lon$th of t~c alimentary canal varies greatly. In three adult giraffes doscnbed by Profc sor Owen it was from 124 to 136 feet long; one dissected in France had this canal 211 feet long; while Dr. Crisp ?l~asurcd ~n~ of the extraordinary length of 254 feet, and s1m1lar vanatwns are recorded in other animals.I The number of ribs varies in many animals. Mr. St. George Mivart says : "In the highest forms of the Primates tho number of true ribs is seven, but in Hylobatcs there arc ~ornetimes eight pairs. In Somnopithccus and Colobus there arc generall! seven, but sometimes eight pa.irs of true ribs. In the Cebidre there arc generally seven or eight pairs, but in Ateles sometimes nine" (Pr;·oc. Zool. Soc., 1 65, p. 56 ). In ~he same paper it is stated that the number of dorsal vortobr::e m .man is normally twelve, very rc.trely thirteen. In the Chimpanzee there are normally thirteen dorsal vertebrro but occasionally there arc fourteen or only twelve. ' Var;·iations in the 'k'llll. Am~ng tho nine adult male Orang -utans, collected hy myself ~n Borneo, the skulls diff'crcd remarkably in size a.nd prop_orti~ns. The orbits Vttricd in width and height, tho cramal r1dgc was either single or double, either much or little developed, and tho zygomatic aperture varied considcrahly in 1 Proc. Zool . • 'uc., 1864, p. 64. |