474 



INDEX 



Sea-horse, 57, 117 

 Sea-mat, 242 

 Sea-pen, 228 

 Sea-snakes, 285 

 Sea-spiders, 114, 397 

 Sea-squirt, 212 

 Sea-urchin, 244 

 Sea-worms, 239 

 Seasonal dimorphism, 394 

 Secretion in leech, 241 



of hag, 272 



of poison gland in snake, 285 



of salivary glands of Collo- 

 calia, 287 



Secretions, internal, 154 

 Segmentation, 309 

 of ovum, 313 

 Segmented worms, 233 

 Selachians, 272 

 Selection, 431 



forms of, 429 

 Self-expression, 185 

 Selvas, 291 

 Semnopithecus, 295 

 Semper, 383, 384 

 Senescence, 330, 332 

 Sense of direction, 137, 171 

 Sensory nerve, 140, 163 

 Serpents, 282 



Sex, evolution of, 301 



- theory of, 302 

 Sex-dimorphism, 253 



-in birds, 290 

 Sexes, contrasted, 303 

 Sexual reproduction, 299 



selection, 103, 107 

 Sheep's stomach, 149 

 Shell of Chiton, 262 

 Shell-gland, 259 



of molluscs, 263 

 Shells of eggs, 307 



of Foraminifera, 178 



- of molluscs, 258 

 Shepherding among animals, 126 

 Shifts for a living, 48-71 

 Shore animals, 6 

 Shore-crab, 335 

 Simocephalus, 406 



Simplest animals, 297 

 Siphonophora, 70, 95, 228 



Sipunculus, 242 

 Sirenia, 293 

 Skate, 273 



sucker, 273 

 Skeleton of Corallium, 231 



- of Crustacea, 249 



of duck, 217 



of insect, 254 



- of mammals, 292 



of sea-urchin, 244 



- of sponges, 226 

 of wings, 226 



Skill in use of materials, 175 

 Skin, functions of, 153 



- of frog, 277 

 Skull of lion, 214 

 Skunk, 59 



Slave-making ants, 87 

 Sleeping-sickness organism, 224 

 Slipper animalcule, 160, 220 

 Slow- worm, 282 



Slug, 260 



Smeathman, 90 



Smith, Geoffrey, 302 



Smooth or unstriped muscle, 



144, 145 

 Snail, 259 

 Snakes, 282 

 Social aspects of heredity, 378 



heritage, 382 



life, advantages of, 97 



- of animals, 72-9 

 forms of, 93 



- organism, 98 

 Society, human, 99 



importance of prehuman, 

 361 



Socrates, 124 



Solitarv bees, 128 



Sollas, "360 



Somatic modifications, 399 



Spadella, 242 



Sparrows, 28 



Spawn of frogs, 279 



Spencer, 76, 98, 182, 201, 321, 

 424 



Spencer's definition of evolu- 

 tion, 425 



Spermatocytes, 310 



Spermatogonia, 308 



