THE DINGO OR WILD DOG OF AUSTRALIA, PERHAPS AN INDIGENOUS WILD SPECIES, PERHAPS A DOMESTICATED DOG THAT HAS GONE WILD OR FERAL 216 Photo: W. S. Berridge, F.Z.S. WOODPECKER HAMMERING AT A COTTON-REEL, ATTACHED TO A TREE 217 THE BEAVER 220 THE THRUSH AT ITS ANVIL 221 Photo: F. R. Hinkins & Son. ALSATIAN WOLF-DOG 226 Photo: Lafayette. THE POLAR BEAR OF THE FAR NORTH 227 Photo: W. S. Berridge. AN ALLIGATOR "YAWNING" IN EXPECTATION OF FOOD 227 From the Smithsonian Report, 1914. BABY ORANG 232 Photo: W. P. Dando. ORANG-UTAN 232 Photo: Gambier Bolton. CHIMPANZEE 233 Photo: James's Press Agency. BABY ORANG-UTAN 233 Photo: James's Press Agency. ORANG-UTAN 233 Photo: James's Press Agency. BABY CHIMPANZEES 233 Photo: James's Press Agency. CHIMPANZEE 238 Photo: W. P. Dando. YOUNG CHEETAHS, OR HUNTING LEOPARDS 238 Photo: W. S. Berridge. COMMON OTTER 239 Photo: C. Reid. SIR ERNEST RUTHERFORD 246 Photo: Elliott & Fry. J. CLERK-MAXWELL 246 Photo: Rischgitz Collection. SIR WILLIAM CROOKES 247 Photo: Ernest H. Mills. PROFESSOR SIR W. H. BRAGG 247 Photo: Photo Press. COMPARATIVE SIZES OF MOLECULES 250 INCONCEIVABLE NUMBERS AND INCONCEIVABLY SMALL PARTICLES 250 WHAT IS A MILLION? 250 THE BROWNIAN MOVEMENT 251 A SOAP BUBBLE (_Coloured Illustration_) 252 Reproduced from _The Forces of Nature_ (Messrs. Macmillan). DETECTING A SMALL QUANTITY OF MATTER 254 From _Scientific Ideas of To-day_. THIS X-RAY PHOTOGRAPH IS THAT OF A HAND OF A SOLDIER WOUNDED IN THE GREAT WAR 254 Reproduced by permission of X-Rays Ltd. AN X-RAY PHOTOGRAPH OF A GOLF BALL, REVEALING AN IMPERFECT CORE 254 Photo: National Physical Laboratory.