Chapter 7 



ADAPTATIONS TO ARHYTHMIC ACTIVITY 



(A) The Twenty-Four-Hour Habit and the Eye 



Of the ways in which natural light can vary, it is the variation of 

 its intensity which is of most importance to animals, and to which they 

 have responded by the most profound of ocular modifications. To adopt 

 the bright hours of day, or the dim ones of night, or to appear indifferent 

 to their alternations, all require adaptations of the eye. These adapta- 

 tions for high sensitivity or for relative insensitivity in turn make pos- 

 sible, or tend to forbid, concomitant adaptations for form-perception 

 and visual recognition on a basis of pattern and color. Animals have 

 had to balance the desirability of a given habit with their ability to use 

 the advantages, and tolerate the disadvantages, which the modifiability 

 of their eyes in the appropriate direction confers or limits. In this and 

 the two succeeding chapters we shall examine the adaptations to illumin- 

 ation-preferences which vertebrate eyes have produced. 



In surveying the visual habits of vertebrates one's attention is natur- 

 ally caught by the extreme conditions of strict diurnality and strict 

 nocturnality, and one tends to suppose that the intermediate condition 

 or arhythmicity, of apparent indifference to night and day, represents a 

 failure to specialize and a lack of adaptation. This is never actually the 

 case — a truly unspecialized and intermediate type of eye would fit its 

 possessor, not for twenty-four-hour vision, but for only the brief periods 

 of morning and evening twilight. The arhythmic animal has to meet a 

 more severe set of requirements than does the rhythmic one of either 

 extreme type, and meets them by combining in one visual organ those 

 adaptations to both bright and dim light which are not mutually exclu- 

 sive. To anticipate the next two chapters, a strongly yellow lens (as in 

 the prairie-dog) goes with diurnality but makes vision in dim light im- 

 possible; and a tapetum lucidum facilitates nocturnality but if non- 



lusible and associated with a super-sensitive retina unprotected by a 

 lit pupil (as in the opossum) , it demands that the animal scrupulously 

 avoid strong light. Obviously, any attempt by an animal to secure twenty- 

 four-hour vision by combining a yellow lens with a tapetum would result 

 in his having wretched vision at any and all times. 



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