164 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



of form-perception, to retain cones as well as the newer rods, the duplex 

 retina as we know it today finally crystallized in a condition which made 

 it possible at last for an animal to become arhythmic if various consider- 

 ations made that desirable. A mechanism for discriminating hues was 

 probably added rather late as a refinement whose first purpose was far 

 from the aesthetic one which color-vision seems, to our anthropocentric 

 minds, to serve (see pp. 463-4) . 



Present-day pure-cone retinae are thus no more primitive than pure-rod 

 ones, for both represent the secondary discard of a cell type for the sake 

 of extreme speciaUzations — which, as always, demand in payment the 

 surrender of plasticity. And, not only have various vertebrates at various 

 times swung from twenty-four-hour capacity toward diurnality or noc- 

 tumality, but they have returned from one extreme through arhythmicity 

 or crepuscularity and even gone on to the opposite extreme. 



Wherever even a few cones have been retained in a rod-rich retina, 

 or a few rods in an almost pure-cone one, manipulations of sensitivity 

 need be only quantitative and are as readily carried out in evolution as 

 an alteration of the ratio of white blood cells to reds. But where the 

 ancestors of a given group retained only one visual-cell type, it might 

 seem impossible for any descendants ever to produce the other. Exactly 

 this has happened, however, and apparently far more often among the 

 reptiles than in any other extant group. Historically, these were the first 

 vertebrates to feel fully the strain of being highly active without benefit 

 of a high body temperature. Not only were they deprived of the warmth 

 of the ancient waters, but they were without the energy-saving buoying 

 effect which a fish enjoys. A little exertion goes a long way when the 

 weight of the body is supported by water, and the tenderness of cooked 

 fish flesh, like that of the disused flight muscles or 'breast meat' of a 

 chicken, is a reminder of the easy lives such muscles lead. 



It is not surprising that in the first terrestrial groups (the stego- 

 cephalians and the reptiles) many sub-groups tended early to develop 

 strong diurnality and pure-cone retinae, counting upon the warmth of 

 the sun to speed metabolism to a degree which would permit of athletic 

 agility in the search for food. Moreover, diurnality was an especially 

 safe habit because of the temporary paucity of enemies on land. But the 

 reptiles have had their heyday and have perforce yielded their place in 

 the sun to the more successful birds and mammals. Most reptiles are 

 still strictly diurnal, but as their enemies have multiplied and their 

 average size has steadily decreased since the days of the dinosaurs, many 



