,
land, distribution of, 397.

Silene, fertility of crosses, 257.

Silliman, Professor, on blind rat, 137.

Skulls of young mammals, 197, 437.

Slave-making instinct, 219.

Smith, Col. Hamilton, on striped horses, 164.

Smith, Mr. Fred.:
on slave-making ants, 219.
on neuter ants, 239.

Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283.

Snap-dragon, 161.

Somerville, Lord, on selection of sheep, 31.

Sorbus, grafts of, 262.

Spaniel, King Charles's breed, 35.

Species:
polymorphic, 46.
common, variable, 53.
in large genera variable, 54.
groups of, suddenly appearing, 302, 306.
beneath Silurian formations, 306.
successively appearing, 312.
changing simultaneously throughout the world, 322.

Spencer, Lord, on increase in size of cattle, 35.

Sphex, parasitic, 218.

Spiders, development of, 442.

Spitz-dog crossed with fox, 268.

Sports in plants, 9.

Sprengel, C. C.:
on crossing, 98.
on ray-florets, 145.

Squirrels, gradations in structure, 180.

Staffordshire, heath, changes in, 72.

Stag-beetles, fighting, 88.

Sterility:
from changed conditions of life, 9.
of hybrids, 246.
laws of, 254.
causes of, 263.
from unfavourable conditions, 265.
of certain varieties, 269.

St. Helena, productions of, 389.

St. Hilaire, Aug., on classification, 418.

St. John, Mr., on habits of cats, 91.

Sting of bee, 202.

Stocks, aboriginal, of domestic animals, 18,

Strata, thickness of, in Britain, 284.

Stripes on horses, 163.

Structure, degrees of utility of, 201.

Struggle for existence, 60.

Succession, geological, 312.

Succession of types in same areas, 338.

Swallow, one species supplanting another, 76.

Swim-bladder, 190.

System, natural, 413.

Tail:
of giraffe, 195.
of aquatic animals, 196.
rudimentary, 454.

Tarsi deficient, 135.

Tausch on umbelliferous flowers, 146.

Teeth and hair:
correlated, 144.
embryonic, traces of, in birds, 451.
rudimentary, in embryonic calf, 450, 480.

Tegetmeier, Mr., on cells of bees, 228, 233.

Temminck on distribution aiding classification, 419.

Thouin on grafts, 262.

Thrush:
aquatic species of, 185.
mocking, of the Galapagos, 402.
young of, spotted, 439.
nest of, 243.

Thuret, M., on