ADVANTAGES, LIMITATIONS; LIGHTLESS HABITATS 209 



(available also, of course, to the nocturnal predator) it imposes some 

 restrictions on diet. Tiny food objects cannot so easily be discerned, and 

 we find nocturnal animals to be relatively gross feeders, cropping vege- 

 tation which they have located by scent, rather than pecking at seeds, 

 and seizing large, unaware prey or motionless nestlings rather than 

 running down minute insects. Where insects do constitute the food, they 

 are not usually caught individually after visual location, but are 'trawled' 

 in numbers, as by the sticky tongue of an ant-eater. Seeds are sought in 

 numbers also— the rodents proverbially prefer their seeds in bunches, 

 as in a head of wheat or an ear of maize. 



The superior visual acuity of the diurnal vertebrate often enables him 

 to maintain an enormous disparity between his armament and the de- 

 fenses of his prey — as when a hawk seizes a garter-snake or a kingbird 

 catches a fly. The nocturnal carnivore must have superior weapons, for 

 he must usually fight on more nearly equal terms with relatively much 

 larger prey. He prefers to catch nocturnal prey at a disadvantage in the 

 daytime, and it is not surprising that carnivorous forms are as often 

 twenty-four-hour animals as strictly nocturnal ones. The very strictest 

 of nocturnality is seen among those preyed-upon animals which are so 

 defenseless that they dare not come out of their hidey-holes even to bask. 

 In this category fall most of the legions of rodents. 



Lightless Habitats and their Conquest — At this point we should 

 give a moment's attention to the fact that in addition to nocturnality 

 Ijy the clock', there are several other dim-light habits of vertebrates 

 which might seem to call for the same ocular modifications : the fossorial 

 habit (as exhibited by the mole, as opposed to forms like the woodchuck 

 which live in a burrow but use it only as a home) ; the cave-dwelling 

 habit (as developed by the permanent residents of caves in contrast to 

 such animals as the bats, which use caves only temporarily; the intern- 

 ally-parasitic habit; the deep-sea habit; and the occupation of very mud- 

 dy waters. 



The habitats involved here are practically or entirely lightless, and 

 the animals which have adopted them have, for the most part, given 

 up any attempt to see and have allowed the eye to degenerate to a tiny, 

 even microscopic vestige, or to vanish altogether (see also pp. 387-405, 

 and Fig. 133). Well-developed eyes, adapted for dim-light vision, are 

 found only in those forms which occasionally venture into one of these 

 habitats for purposes other than mere temporary concealment; and out- 



