1i? THE NEW EVOLUTION ^^^^ 



or less scattered over the surface of the body. In the 

 birds and in some reptiles which have the body unusu- 

 ally well insulated from the surrounding air there is a 

 special heat recording apparatus called the pecten 

 within the visual eye. 



It is obvious that good vision is without value to 

 any creature with a bodily structure of such a nature as 

 to render it incapable of rapid locomotion. It is of no 

 advantage to be able to see food if a creature cannot 

 reach it, or to be able to see an enemy if escape from 

 that enemy is impossible. So all fixed animals, like 

 corals (fig. 80, p. 143), sea-pens (fig. 77, p. 143), hy- 

 droids, polyzoans (figs. 67, 68, p. iii), clams and 

 oysters, are either wholly blind or have eyes without 

 a visual function. Creatures with feeble locomotor 

 powers, like caterpillars, scallops, snails (fig. 51, 

 p. 97), sea-urchins (fig. 41, p. 71) and starfishes (fig. 

 41, p. 71), have eyes with at most a very limited 

 capacity for vision, and are often blind. 



Perfect visual eyes are of advantage only to animals 

 able to profit by an acute visual capacity, which means 

 animals whose structure renders rapid locomotion pos- 

 sible. Such creatures are extremely few, and are 

 entirely confined to three of the major groups, the 

 vertebrates, the arthropods and the mollusks. 



All vertebrates are capable of locomotion which, 

 because of their large size, is always relatively rapid. 

 But not all of the vertebrates are capable of sight. 

 Blind forms with defective or quite sightless eyes are 

 found among the mammals, reptiles, amphibians and 

 fishes, though not among the birds. 



" [78] 



