EVOLUTION— PYCRAFT 235 



Dr. Andrews regarded Moeritherium as the direct ancestor of all 

 the elephants. But Professor Osborn dissented, holding it to be off 

 the direct line of descent. Be this as it may, the animal he desig- 

 nates as the ancestor is not so very different in essentials. This Avas 

 Phiomia^ of which there were four species of increasing size. Their 

 incisors closely resembled those of Moeritherium^ but those of the 

 lower jaw were gradually borne forward by the extension of the jaw, 

 so as to keep the teeth in touch with the ground, the lengthening 

 of the neck being inhibited by the intensive use of the jaw as a dig- 

 ging instrument. Palemastodon marks an important stage in this 

 development. Herein the upper tusks are beginning to turn forward. 

 We have passed now from the Mid-Eocene to the Lower Oligocene, 

 a period representing several million years. This process of the 

 lengthening of the lower javv", bearing the digging teeth, culminated 

 in Trilophodon wherein it attained a length of 6 feet 7 inches, the 

 teeth being borne at the end of a long beam ! 



To keep pace with the lengthening jaw, the upper lip became more 

 and more elongated, passing from a tapirlike snout into a gradually 

 lengthening proboscis. With such a very mobile, and efficient grasp- 

 ing organ, which enabled browsing to replace digging, the reduc- 

 tion in the length of the lower jaw started, and the typical elephants, 

 fossil and recent, came into being. 



But this is not the whole story. There were many divergent 

 branches. In some the lower jaw expanded at its tip, giving rise to 

 the "shovel-toothed" mastodons, in others as in Deinotheres^ the lower 

 incisors, borne on a relatively short jaw, turned downward, to form 

 a pair of great hooks. 



During all this time the cheek teeth, or "grinders", were becoming 

 reduced in number, but larger and larger and more and more com- 

 plex, well seen in the African and Indian elephants of today. Wlien 

 one comes to realize that this pedigree of the elephants covers a 

 period of some 30,000,000 years, it becomes apparent that the effects 

 of use and disuse are ponderously slow. But they are nevertheless 

 very real. 



Here we have striking sequences of growth regulated by the stren- 

 uous use of the jaw in digging which inhibited the lengthening of 

 the neck. Later, this inhibition was enforced by the increasing 

 weight of the head, due to the progressive enlargement of the tusks 

 and trunk and their consequent weight. 



As an example of the effects of intensive use in the history of the 

 evolution of a race, this of the elephant, and the horse, is the most 

 complete. 



And now let me pass to a brief summary of "ontogeny" — the 

 gradual development of the body of the individual from the em- 



