crossed fuci, 258.

Thwaites, Mr., on acclimatisation, 140.

Tierra del Fuego:
dogs of, 215.
plants of, 374, 378.

Timber-drift, 360.

Time, lapse of, 282.

Titmouse, 183.

Toads on islands, 393.

Tobacco, crossed varieties of, 271.

Tomes, Mr., on the distribution of bats, 394.

Transitions in varieties rare, 172.

Trees:
on islands belong to peculiar orders, 392.
with separated sexes, 99.

Trifolium pratense, 73, 94.

Trifolium incarnatum, 94.

Trigonia, 321.

Trilobites, 306.
sudden extinction of, 321,

Troglodytes, 243.

Tucutucu, blind, 137.

Tumbler pigeons:
habits of, hereditary, 214.
young of, 446,

Turkey-cock, brush of hair on breast, 90.

Turkey:
naked skin on head, 197.
young, wild, 216.

Turnip and cabbage, analogous variations of, 159.

Type, unity of, 206.

Types, succession of, in same areas, 338.

Udders:
enlarged by use, 11.
rudimentary, 451.

Ulex, young leaves of, 439.

Umbelliferae, outer and inner florets of, 144.

Unity of type, 206.

Use:
effects of, under domestication, 11.
effects of, in a state of nature, 134.

Utility, how far important in the construction of each part, 199.

Valenciennes on fresh-water fish, 384.

Variability of mongrels and hybrids, 274.

Variation:
under domestication, 7.
caused by reproductive system being affected by conditions of life, 8.
under nature, 44.
laws of, 131.

Variations:
appear at corresponding ages, 14, 86.
analogous in distinct species, 159.

Varieties:
natural, 44.
struggle between, 75.
domestic, extinction of, 111.
transitional, rarity of, 172.
when crossed, fertile, 267.
when crossed, sterile, 269.
classification of, 423.

Verbascum:
sterility of, 251.
varieties of, crossed, 270.

Verneuil, M. de, on the succession of species, 325.

Viola tricolor, 73.

Volcanic islands, denudation of, 284.

Vulture, naked skin on head, 197.

Wading-birds, 386.

Wallace, Mr.:
on origin of species, 2.
on law of geographical distribution, 355.
on the Malay Archipelago, 395.

Wasp, sting of, 202.

Water, fresh, productions of, 383.

Water-hen, 185.

Waterhouse, Mr.:
on Australian marsupials, 116.
on greatly developed parts being variable, 150.
on the cells of