_Eunectes sticticus_, Linn., unaffected by climate, 31. Euphorbias, their concentration in Southern Africa, 142. _Eurygnathus Latreillei_, Lap., variety of, on one of the Madeira Islands, 88, 109. Exceptions, not be allowed to negative a law, 72, 73. Extinction of species, as indicated in the Madeiran _Helices_, 131; the only cause by which genera may be abruptly defined, 176. Forbes, Prof. E., on the origin of the British animals and plants, 130; his epochs of migration of the British animals and plants, 136; on the existence of a former Atlantic continent, 137. Forests, the hindrance which they offer to insect-diffusion, 154. "Fortunate Islands" of the ancients, probably the Canarian group, 141. Galapagos, fauna of, 23. Genera, the nature of, often misunderstood, 160; a familiar explanation of, 160, 161, 162; cannot be abrupt except from accident, 169; how to be defined, 169; the types of, usually situated towards the centres of the several groups, 170; the types of, usually evenly balanced in their structural characters, 171, 172; may be abruptly defined from accidental causes, 176, 177. Generic areas, an important feature throughout Nature, 130, 141, 184. Geology, a necessary item in the study of insect-diffusion, 113. Germanic plains, the, probably a primary area of diffusion, 130. _Gerris_, on the development of the wings of, 100. Gould, Mr., on the Swallows of Malta, 102. _Gymnaetron_, blood-red dashes characteristic of, 62. _---- Campanulae_, Linn., its smallness on the Cornish coast, 58. _---- Veronicae_, Germ., a variety of _G. niger_, 62. _Hadrus illotus_, Woll., perhaps a form of _H. cinerascens_, 66. _Haliplus obliquus_, Gyll., dark state of, in Ireland, 67. _Haltica exoleta_, Fabr., its variability on the coast, 59. Harcourt, Mr., on the discovery of Madeira, 49, 50. _Harpalus vividus_, Dej., changes to which it is subject, 67, 68, 69; variable in the connateness of its elytra, 96, 97. _Hegeter_, its maximum attained in the Canaries, 120. _---- elongatus_, Oliv., its migration from the Canaries, 120; of a more adaptive nature than its allies, 121. _---- latebricola_, Woll., its occurrence in the Salvages, 120. _Helices_, have often two distinct states, 106; many of them representative in the Madeira Islands, 128, 129; those in the Madeiras chiefly of slow migratory powers, 130, 131. _Helix attrita_, Lowe, its local character, 132. _---- Bowdichiana_, Fer., perhaps a gigantic state of _H. punctulata_, 106. _---- calculus_, Lowe, sedentary nature of, 132. _Helix commixta_, Lowe, sedentary nature of, 132. _---- coronata_, Desh., its peculiarity to Porto Santo, 128; its occurrence beneath the surface of the ground, 131. _---- coronula_, Lowe, its peculiarity to the Southern Dezerta, 128.