Salamandra, maculosa and atra, 182,199,203 spotted and striped, 207 geographical variation of, 208 Segmentation, nature of, 63 simulated mechanically, 64 compared with rippling, 65 analogies with, 68 Segmentation of normally unsegmented structures, 38 Selection, Natural, an insufficient cause of definiteness of types, 17,134,142 Sempervivum, 250 Serial Homology, the true nature of, 62,66 Setina, Alpine varieties, 181 Sex of Twins, 44 Sex-factors, possible coupling of, 111 Sexual characters, variation in, 119 et seq. Siamese twins, 44 Silky Fowl, 84,85 Simocephalus, changed by environment, 218 Sinistral forms, 33-34 Situs transversus, 43 Skate's jaws, variation in, 38 Sloths, vertebral variation, 68 Species, conceptions of, 3,94,99,240,245 allied, distribution of, 185 alternative uses of the term, 245 Specific difference, universality of, 12 of organisms compared with those of inorganic materials, 15 failure of theory of Selection to explain, 18,134,247 Sphyropicus varius, 149,156 Spilosoma lubricipeda, varieties of, 181 Zatima, Heligoland form, 181 Spinal nerves, segmentation of, 67 Sporadic variation, 131,134,248 Squashes, polymorphism of, 100 Staphylococcus pyogenes, variation in, 213 Sterility of hybrids, in general, 233 in Lychnis hybrids, 20 et seq. in crossing forms of Draba, 243 Significance of, 244 Self, 238 Stilt, 234 Stocks, 84,104,113 Striped varieties, 57 Substantive variation, 84 Subtraction-stages, 93 Supernumerary limbs, 72-76 Sweet pea, variation of, 91