CONTENTS xi 



PAGES 



does evolution, according to Darwin, depend upon chance? p. 132 Pro- 

 fessor Weldon's theory, p. 134 Positions of stability, p. 136 Heredity 

 limits the range of variations, p. 137 Convergent evolution, p. 142 

 Correlation, p. 143 Sequence of variation, p. 143 Can we attribute the 

 evolution of all species to an inherent tendency? p. 143 Easiness of 

 adaptation, p. 144 Nature of environment, p. 145 Several possible 

 environments, p. 146 Variety of possible adaptations to exactly similar 

 environments, p. 147 The environment, properly defined, is rigid, p. 148 

 Summary, p. 149. 



IX. CONSCIOUSNESS . . . . . . .149-152 



Consciousness assumed in the lowest organisms, p. 150 "The all- 

 sufficiency of Natural Selection," p. 151. 



X. SUMMARY ........ 152-155 



Chapter V 

 SEXUAL SELECTION 



Spermatozoon and ovum, p. 156 Secondary sex characteristics, p. 157 

 Need of a cumulative principle, p. 160 Darwin's theory, p. 160 

 A difficulty in Darwin's theory, p. 163 M. Stolzmann's theory, p. 164 

 The explanation of the difficulty, p. 165 Fine plumage the concomitant 

 of vigour, p. 165 Monogamous species, p. 166 Recapitulation, p. 167 

 Dr Russel Wallace, p. 167 Sexual selection co-operates with Natural 

 Selection, p. 168 The giraffe's neck, p. 169 Migratory birds, p. 170 

 Circumstances under which polygamy arises, p. 172. 



Chapter VI 

 ISOLATION 



I. ISOLATION OF ANIMAL SPECIES ..... I 7S" I ^S 



Importance of isolation, p. 175 Clannish isolation p. 176 Inter- 

 sterility, p. 176 Romanes and Mr Gulick, p. 177 Land molluscs of the 

 Sandwich Islands, p. 177 Recognition marks, p. 180 Isolation among 

 birds, p. 181 Distinctive song and distinctive plumage often alterna- 

 tive, p. 182 Vocal power and migration, p. 184. 



II. ISOLATION OF SPECIES OF FLOWERING PLANTS . . . 185-195 



Darwinism must somehow account for flowers, p. 186 Flowers and 

 insects, p. 187 Reciprocity, p. 188 Colour sense, p. 189 The constancy 

 of bees, p. 190 Intersterility between species, p. 192 Varieties of garden 

 flowers, p. 192 Varying individuals, p. 193 Insect workers under these 

 conditions, p. 193 Sterility and infertility, p. 194 Wind- and self-fertilised 

 flowers, p. 194 Summary, p. 195. 



