84 SCIENCE PROGRESS 



in the aye-aye. Still it was probably insects that first attracted 

 them into the trees in search of nitrogen. 



As for obtaining nitrogen direct from leaves and nuts it is 

 probable that these parts of the plant were not so rich in 

 nitrogen in the Eocene as they are at present. Arboreal forms, 

 such as squirrels, lemurs, and monkeys, eat birds, mice, and 

 eggs, a habit which shows that the protein problem is not 

 easily solved on a purely vegetable diet ; and the primate type 

 of the alimentary canal combined with the large eye points 

 to birds and not to leaves and seeds. It may be that the 

 attacks of such small animals brought about the roosting 

 instinct of many birds, to which reference will be made at a 

 later stage. It is also of interest to note that at the present 

 day some of the lemurs are tamed and employed to catch 

 birds in the trees at night. 



The Upper Eocene Change 



It was probably during the Eocene that the lemurs, New 

 World monkeys, and Old World monkeys separated from one 

 another. The profound differences between the three indicate 

 that the proto-primates were still very generalised at the time 

 when this separation took place. For the most part the descend- 

 ants of all three branches have remained small and arboreal. 

 They have each developed their own type of teeth, of hands, 

 of the feet and of the tail. Concurrent with the evolution of 

 these organs each of these branches developed instincts along 

 its own special lines. Proto-primate had probably eleven ribs 

 and some seven or eight lumbar vertebrae, and hence his descend- 

 ants have been able to add to the ribs and to adjust the lumbar 

 and sacral vertebras according to the needs of bony protection, 

 and of strength or flexibility of the backbone. 



It was probably in the Upper Eocene that the branch of 

 the Old World monkeys which gave rise to the apes and man 

 came to the ground. Perhaps it had grown too heavy for the 

 small branches of the trees, and birds had become too wary 

 to be caught as easily as of old. This branch had peculiarities 

 which already marked it off from most of the monkeys, and 

 had retained many primitive features. It had small conical 

 canine teeth, small bones formed the capsule of the olfactory 

 nerve, a rounded head perhaps resembling that of the young 

 chimpanzee, a vermiform appendix, and probably a very short 

 tail. The relative length of the arms and legs was perhaps 

 the same as has been retained by the chimpanzee ; let us 

 suppose that it had an opposable thumb and a clasping foot, 

 the condition of the two extremities being the opposite of that 

 found in the American monkeys. 



