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 (3) The male generative pores varied in position from 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 Darwin's unpublished MSS. "In some species of Shrews (Sorex) and in some field-mice (Arvicola), the Rev. L. Jenyns (Ann. Nat. Hist., vol. vii. pp. 267, 272) found the proportional length of the intestinal canal to vary considerably. He found the same variability in the number of the caudal vertebræ. In three specimens of an Arvicola he found the gall-bladder having a very different degree of development, and there is reason to believe it is sometimes absent. Professor Owen has shown that this is the case with the gall-bladder of the giraffe." Dr. Crisp (Proc. Zool. Soc., 1862, p. 137) found the gallbladder present in some specimens of Cervus superciliaris while absent in others; and he found it to be absent in three giraffes which he dissected. A double gall-bladder was found in a sheep, and in a small mammal preserved in the Hunterian Museum there are three distinct gall-bladders. The length of the alimentary canal varies greatly. In three adult giraffes described by Professor Owen it was from 124 to 136 feet long; one dissected in France had this canal 211 feet long; while Dr. Crisp measured one of the extraordinary length of 254 feet, and similar variations are recorded in other animals.1 The number of ribs varies in many animals. Mr. St. George Mivart says: "In the highest forms of the Primates, the number of true ribs is seven, but in Hylobates there are sometimes eight pairs. In Semnopithecus and Colobus there are generally seven, but sometimes eight pairs of true ribs. In the Cebidae there are generally seven or eight pairs, but in Ateles sometimes nine" (Proc. Zool. Soc., 1865, p. 568). In the same paper it is stated that the number of dorsal vertebræ in man is normally twelve, very rarely thirteen. In the Chimpanzee there are normally thirteen dorsal vertebræ, but occasionally there are fourteen or only twelve. Variations in the Skull. Among the nine adult male Orang-utans, collected by myself in Borneo, the skulls differed remarkably in size and proportions. The orbits varied in width and height, the cranial ridge was either single or double, either much or little developed, and the zygomatic aperture varied considerably in 1 Proc. Zool. Soc., 1864, p. 64. size. I noted particularly that these variations bore no necessary relation to each other, so that a large temporal muscle and zygomatic aperture might exist either with a large or a small cranium; and thus was explained the curious difference between the single-crested and the double-crested skulls, which had been supposed to characterise distinct species. As an instance of the amount of variation in the skulls of fully adult male orangs, I found the width between the orbits externally to be only 4 inches in one specimen and fully 5 inches in another. Exact measurements of large series of comparable skulls of the mammalia are not easily found, but from those available I have prepared three diagrams (Figs. 14, 15, and 16); in order to exhibit the facts of variation in this very important organ. The first shows the variation in ten specimens of the common wolf (Canis lupus) from one district in North America, and we see that it is not only large in amount, but that each part exhibits a considerable independent variability.1 In Diagram 15 we have the variations of eight skulls of the Indian Honey-bear (Ursus labiatus), as tabulated by the late Dr. J. E. Gray of the British Museum. For such a small number of specimens the amount of variation is very large-from one-eighth to one-fifth of the mean size,—while there are an extraordinary number of instances of independent variability. In Diagram 16 we have the length and width of twelve skulls of adult males of the Indian wild boar (Sus cristatus), also given by Dr. Gray, exhibiting in both sets of measurements a variation of more than one-sixth, combined with a very considerable amount of independent variability.2 The few facts now given, as to variations of the internal parts of animals, might be multiplied indefinitely by a search through the voluminous writings of comparative anatomists. But the evidence already adduced, taken in conjunction with the much fuller evidence of variation in all external organs, leads us to the conclusion that wherever variations are looked for among a considerable number of individuals of the more 1 J. A. Allen, on Geographical Variation among North American Mammals, Bull. U. S. Geol. and Geog. Survey, vol. ii. p. 314 (1876). 2 Proc. Zool. Soc. Lond., 1864, p. 700, and 1868, p. 28. |