THE PERCEPTION OF LIGHT 603 



evening. Such animals depend essentially on senses other than vision 

 in their activities ; form vision need only be crude for merely a hazy 

 outline is visible, and colour vision would appear to be useless. 



(3) ARHYTHMic ANIMALS, the cycs of whicli have sufficient 

 plasticity to adapt themselves either to bright or dim illumination. 



It can be assumed that diurnality was the primitive state in 

 Vertebrates which presumably evolved initially in shallow waters. 

 Nocturnality has probably been developed for two reasons — to escape 

 from danger and to obtain food. It is likely that a lightless habitat 

 was first sought as a refuge from stronger and more powerful enemies, 

 whether it be the abyss of the seas, the recesses of a cave, the shelter 

 of a stone or a burrow in the earth, or merely the protection afforded 

 by the darkness of night. When in the early Cenozoic age the littoral 

 or pelagic seas became increasingly populated by larger and still larger 

 predators, in order to survive more and more of the defenceless type 

 of fishes sought refuge in the deej>er and darker depths where light 

 becomes gradually dimmer and is ultimately extinguished ; to adapt 

 itself to this environment the eye became more and more specialized to 

 pick up the small amount of light available and vision necessarily 

 became more crude. As always happens, however, the security of 

 these refugees would not last, for predators would follow in increasing 

 numbers from the highly populated pelagic zone to feast with less 

 competition on the untouched store of food available in the darker 

 waters. Thus the primitive Cyclostomes are diurnal (except Geotria) ; 

 the Selachians, the Chondrosteans, the Dipnoans and the Coelacanths 

 have all become nocturnal ; but the more highly developed Holosteans 

 are diurnal and the eminently specialized Teleosteans which have 

 succeeded in establishing themselves as over-all masters of the seas are 

 of various habits as if to suit their convenience, and some of them, 

 such as the belligerent pike, are highly diurnal. 



Similarly, although the first venturers on land must have had a 

 safe and easy time in their new environment rich in vegetable and 

 insect food and relatively empty of powerful enemies, the evolution of 

 more specialized types with a more efficient armature and more active 

 habits forced many of the primitive species to seek lightless surround- 

 ings or the cover of night in order to survive ; the penalty for failure 

 in this adaptation was usually extinction. Apart from the frogs, all 

 Amphibians which have survived are therefore markedly nocturnal or 

 secretive in habit ; apart from the turtles, all Reptiles which have 

 survived are also nocturnal except the majority of the recently developed 

 lizards and their off-shoot, the still more modern snakes, many of 

 which, initially nocturnal and burrowing, have acquired a new 

 diurnality. Freed from the danger of land animals in their new 

 aerial environment, most Birds can afford to be diurnal, although 



