xxix. 4 GENERA OF ELEPHANTS 709 



related Dendrohyrax live in the trees. Earlier hyraxes were sometimes 

 as large as small horses (*Megalohyrax from the Oligocene). 



4. Elephants. Order Proboscidea 



The late Henry Fairfield Osborn, one of the greatest zoologists of 

 his time, devoted a great part of a long working life and the large 

 resources available to him at the American Museum of Natural 

 History to the study of the evolution of elephants. It cannot be said 

 that he was able as a result of all this study to draw conclusions that 

 have revolutionized biology, and this failure is in a sense a measure 

 of the immaturity of our science. The elephants have shown great 

 and relatively rapid changes in recent geological times and have left 

 abundant remains, especially of their large and hard teeth. We may 

 therefore take knowledge about evolution of elephants as a fair example 

 of the most that can be known of the evolutionary processes in large 

 mammals ; if the study of this great mass of material leaves us in a state 

 of confusion rather than of certainty we shall be warned to suspect the 

 apparent clarity of other alleged evolutionary sequences, and to dis- 

 trust dogmatic statements about the 'causes' of evolutionary change. 



The two existing types of elephant, referred to distinct genera, live 

 still in considerable numbers in Africa (Loxodonta) and Asia (Elephas). 

 They are survivors of a much larger population, reaching its greatest 

 variety in Pliocene times. The essential feature of the type is the great 

 size (11 1 ft high in Loxodonta) and the presence of a special food- 

 collecting system able to gather enough raw material to support such 

 a large living mass. Of the various factors influencing the optimum 

 size for a given animal type, all those favouring increase must be 

 present in the ingredients of elephant life. Elephants are larger than 

 any other land animals, living or extinct, except perhaps the huge 

 Oligocene rhinoceros *Baluchitherium and some of the largest dino- 

 saurs (if these were indeed terrestrial). 



The basal metabolism of a mammal increases only with about the 

 two-thirds power of its weight, so that larger animals need, on this 

 account, relatively less food. But, as Watson (1946) has pointed out, 

 the output of energy by the muscles is proportional to the weight of 

 the muscle; the total intake needed is therefore proportional to some 

 power between two-thirds and the first power of the weight of the 

 whole, being 'larger the greater and more continuous the activity of 

 the animal'. Elephants, as he further comments, are very active when 

 wild, their playfulness and strength are proverbial, and often a 

 nuisance to man. They manage to collect their food with sufficient 



