604 



THE EYE IN EVOLUTION 



Primitive 

 Ampliibian 



Primitive 

 Reptile 



in their search for food many have become crepuscular and a few, 

 particularly the owls, essentially nocturnal. 



Flightless birds are therefore in a peculiarly precarious position and many 

 of them have been exterminated : the nioas in New Zealand on the arrival of 

 man (Fig. 737) ; the dodo of Maviritius on the arrival of mammals (Fig. 738) ; 

 while the kiwi of New Zealand, even althovigh taking refuge in nocturnality, is 

 now almost extinct (Fig. 739). 



Similarly in their search for safety from the larger Amphibians 

 and Reptiles which inhabited the earth at the time of their emergence, 

 the early Mammals were nocturnal or crepuscular — all the Mono- 

 tremes, all the smaller and more primitive Marsupials and most of 

 the primitive Placentals. Among these last, only a few have acquired 



Fig. 737.— The Moa. 



Fig. 738.— The Dodo. 



Fig. 739.— The Kiwi. 



Tupaia 



Castor 



diurnality, particularly the tree-shrews {Tupaia) among the Insecti- 

 vores, and the squirrels (except the flying squirrels) among the Rodents ; 

 to these the diurnal habit was possible owing to the relative safety 

 of their arboreal life and its acquirement was probably stimulated by 

 the necessity for constant agility in their environment. The other 

 diurnal Rodents are few — the beaver, Castor, the cavy, Dolichotis, and 

 the pika, Ochotona; the coney, Procavia, is of the same habit. The small 

 Carnivores, except the viverrid, Cynictis, and the suricate, Suricata, 

 are also primarily nocturnal or crepuscular, but the larger Marsupials 

 and the Ungulates and the larger Carnivores have become arhythmic, 

 the first two emerging into the daylight because of the safety provided 

 by their agility and fieetness, the last because of their ferocity and the 

 excellence of their weapons of offence. As did their ancestors, the 

 Insectivores, the early Primates found safety in nocturnality ; 

 practically all the Prosimians are nocturnal except some members of 

 the family of lemurs, ^ but having acquired safety in their agility and 

 intelligence, all the Simians except the night-monkey Nyctipithecus, 

 are diurnal or arhythmic. 



^ A few species are diurnal such as Propithecus, Inclris, and Hapalemur. 



