tribution of, 204-248;
extinct and living allied on same areas, 213;
life of, preserved by natural selection, 264-270;
not room for more than one rational, 344;
characters of, 274-276, 286-295, 374-376;
inter-sterility of allied, 374-376;
mutual ministration of alleged, 445, 446.

Specific characters, _see_ Characters.

Speculation, method of, 3-9.

Spencer, Herbert, on reproduction as discontinuous growth, 105, 106;
on use-inheritance, 253-256;
his failure to conceive the idea of natural selection, 257.

Spermatozoa, 123, 126-128.

Spiders, in primary formations, 163;
courtship of, 388, 389.

Sponges, 122, 139, 140.

Spontaneous, Darwin's use of the term, 340.

Spores, 115.

Squirrels, flying, 355.

Sterility, _see_ Infertility.

St. Helena, 231-234, 236-237.

St. Hilaire, 4.

Stick-insect, 322.

Stoat, 318.

_Strombus accipilrinus_, 201.

_Strombus Leidy_, 201.

Struggle for existence, 259-270.

Subjective, methods, 6.

Survival of the fittest, 335. _See also_ Natural selection.

Swim-bladder of fish, 154, 354.

Symbiosis, 269.

Syme, David, on the theory of natural selection, 340, 341.

T.

Tail, types of, in fish and birds, 169-173.

Tasmanian wolf, dentition of, 39.

Teeth, of Tasmanian wolf, 39;
molar, of man, compared with those of apes, 92-94;
palaeontology of horses', 189-191.

Temperature, sense of, probable origin of that of sight, 353, 354.

Tennyson, 266.

Tibiae, flattening of, 95, 96.

Tissue-cells, _see_ Cell.

Toes, 79, 80; _see also_ Feet.

Tomes, C. S., on molar teeth of man and apes, 94.

_Torpedo_, 365, 367.

Tortoise, embryology of, 152, 154.

_Toxopneustes variegatus_, and _T. lividus_, 122.

Transport of organisms, means of, 207, 216-218.

Tribal fitness, as distinguished from individual, 267-269.

Trout, ovum of, 122.

Turtle, eye of, 75.

Tylor, Alfred, on colouration of animals, 448-450.

Type, preserved by natural selection, 264-269;
improvement of, by natural selection, 269, 270;
prophetic, 272, 351-362.

Types, as simple and generalized, 33.

U.

Unicellular organisms, 104.

_Uraster_, 138.

Utility, of specific characters, 274, 275;
of incipient characters, 351-363;
of electric organs, 365-373.

V.

Variation, in relation to natural selection, 263, 335-340, 377.

Verification, 6-9.

Vertebral column, embryology of 145, 146;
palaeontology of, 192, 193.

Vertebrated animal,