20 AN INTRODUCTION TO MODERN GENETICS 



FIG- PAGE 



88. Plus-minus variations 187 



89. Expressivity and environment 189 



90. Diffusion and pattern 194 



91. Ephestia patterns 195 



92. Genetic and environmental effects on patterns 196 



93. Feather patterns 199 



94. Variations in manifestation 200 



95. Polydactyly 203 



96. Combs in fowls 204 



97. Proboscipedia in Drosophila 205 



98. Induction of gamete sexuality 209 



99. Relative sexuality 211 

 100. Intersexes in Lymantria 217 

 loi. Intersexuality and the switch-over 218 



102. Intersexes in Drosophila 220 



103. Development of sexual characters 222 



104. Sex inductors in amphibia 226 



105. Parabiotic twins in amphibia 227 



106. Sex determination in Habrobracon 229 



107. Gryphaeas 243 



108. Ammonites 246 



109. Graptolites ^ 247 

 no. Age and area 249 



111. Centres of origin 251 



112. Primula kewensis 257 



113. Species hybrids in Nicotiana 259 



114. Wheat hybrids 260 



115. Spartina Townsendii 260 



116. Crepis artificialis 262 



117. Drosophila species 265 



118. Comparative chromosome maps in D. melanogaster and 266 



simulans 



