210 ADAPTATIONS TO NOCTURNAL ACTIVITY 



side of the vertically-wandering fishes and whales these are very few 

 indeed. There are many other exceptions constituted by the deep-sea 

 fishes, most of which have enormous eyes whose retention and perfection 

 we can safely attribute to the timely invention of light-producing organs 

 by deep-sea animals. There is some point to a retention of a sense of light 

 and darkness by subterranean forms so that they may be aware when 

 their burrows have been broken into by the weather or by other animals. 

 Such animals, like the moles, marsupial moles, Spalax, and the fossorial 

 reptiles always have enough of an eye to make this much Vision' possible. 

 But the strictly cavernicolous vertebrates, all of them fishes or salaman- 

 ders, have only microscopic, completely non-functional eyes. Of the two 

 dozen or more cave-dwelling species of fishes, only two or three ever (as 

 individual variations) exhibit useful eyes, and in only one of these (the 

 Mexican Anoptichthys jordani) do the eyes vary from zero to complete 

 normality. The same degree of degeneracy as in cave fishes is seen in the 

 parasitic hag-fishes, which 'burrow' — in the bodies of their prey! 



As for the muddy-water problem: several kinds of gobies and at 

 least one mammal (the fresh-water dolphin Platanista gangetica, swim- 

 ming through the roiled waters of the great Indian rivers), have given 

 it up as an impossible job. The eye of Platanista has 'gone bad' in a 

 unique way — this is the only vertebrate with a macroscopic eye which 

 lacks all traces of a lens. In such limicolous gobies as Austrolethops and 

 Trypauchen, and in the sole Typhlachirus, the entire eye is minute or 

 quite obsolescent. In general, the fishes of silty rivers, as in our Great 

 Plains, have somewhat undersized eyes which are useful only close to 

 the surface, where alone there is adequate light. The fishes of the peculiar 

 Lake Balaton have however made a valiant effort to cling to vision 

 despite the quasi-opacity of the water in which they swim (see p. 236). 



The Eye as a Whole — It was hinted earlier (p. 172) that nocturnal 

 animals, as well as diurnal ones, have a special need for a large eye. The 

 need is a very direct one in the case of a diurnal eye: to enlarge the 

 image. The reason why large eyes are desirable for a nocturnal animal is 

 a little more complicated. It is not at all for the improvement of resolving 

 power — a whale eye the size of a baseball has but 2% of the resolving 

 power of the human eye, due to its tremendous retinal summation. 



If we could be watching an animal in the process of evolving nocturn- 

 ality, we might feel impelled to advise him to enlarge his eyes "so more 

 light can enter them." But on second thought we should realize that this 



