The diversity of animal life 177 The evolution theory 181 Natural selection: not to be used as a magic formula 183 Panmixia and disuse 189 Sexual selection or preferential mating 197 Use and disuse 209 The nature of variations 216 The inheritance of variations 223 The origin of variations 231 Summary and conclusion 241 CHAPTER VII. THE SENSES OF ANIMALS. The primary object of sensation 243 Organic sensations and the muscular sense 244 Touch 245 The temperature-sense 249 Taste 250 Smell 257 Hearing 261 Sense of rotation or acceleration 269 Sight 273 Restatement of theory of colour-vision 278 Variation in the limits of colour-vision 281 The four types of "visual" organs 293 Problematical senses 294 Permanent possibilities of sensation 298 CHAPTER VIII. MENTAL PROCESSES IN MAN. The physiological aspect 302 The psychological aspect 304 Sensations: their localization, etc. 306 Perceptual construction 312 Conceptual analysis 321 Inferences perceptual and conceptual 328 Intelligence and reason 330 CHAPTER IX. MENTAL PROCESSES IN ANIMALS: THEIR POWERS OF PERCEPTION AND INTELLIGENCE.