DIURNALITY, ACUITY, AND FOOD 171 



have become nocturnal to avoid predators. Predators have in turn be- 

 come nocturnal to continue to find food easily. To escape the nocturnal 

 predators, prey animals have again become diurnal. Those species and 

 groups which could not invert their habits at need were doomed unless, 

 by sheer weight of numbers, by phenomenal fecundity, they were able to 

 compensate (as species) for the enormous losses of ill-equipped indi- 

 viduals. 



The Eye as a Whole — For an eye to mediate sharp vision, an essential 

 requirement is a large retinal image. The greater the number of visual 

 cells over which the image is spread, the greater the resolution of the de- 

 tails of the image. The histology of the retina is a very important factor 

 but, after all, it can only say the last word in the story the eye tells the 

 brain. There are strict limits to the fineness of the receptor mosaic, and 

 its performance is in turn limited by the size of the image presented to it 

 by the dioptric apparatus. 



The simplest way to gain a large image is to have a large eye; and 

 'large' here refers to absolute, not relative, size; for whereas with other 

 organs of the body it is relativity to each other that determines adequacy 

 of size, the eye is essentially an optical instrument and obeys the laws of 

 inter-organ proportioning only grudgingly, disobeying them entirely 

 whenever, with impunity, it can. Biologists tend to overlook this fact, and 

 frequently remark of large animals, such as the whales, that "their eyes 

 are so small in proportion that they must be just about useless"— forget- 

 ting that the world looks the same size to a whale, a man, and a mouse. 

 They all see as much, but not as well. Were a squirrel as big as a horse, 

 it would have an eye as big as a horse's; but that is not to say that if a 

 horse were as small as a squirrel, it would see as well with an eye propor- 

 tionately small. The squirrel would, on the other hand, be much better 

 off with eyes as big as a horse's — if it had room for such eyes in its head. 



The big reason for this fact — that it is absolute rather than relative size 

 which, ceteris paribus, determines visual acuity — is that the absolute 

 dimensions of retinal elements vary within only narrow limits however 

 large or small the eye may be. Tripling the diameter of the eyeball does 

 not entail tripling the diameter of a cone visual cell. Rather, it results in 

 a tripling of the number of visual cells in a given linear distance on the 

 retina. The image is then three times as broad, and visual acuity is en- 

 hanced threefold. 



In practice it is only exceptionally that high visual acuity can be gained 



