640 THE EYE IN EVOLUTION 



ACCOMMODATION IN VERTEBRATES 



While an emmetropic static refraction is necessary for the attain- 

 ment of a high degree of visual acuity, the capacity to adjust the 

 optical system for near or distant vision is almost equally important, 

 particularly in such activities as the capture of prey. In an amphibious 

 life if any adjustment to the two environments is attempted the 

 importance of accommodation is still greater owing to the difference 

 in refractivity between water and air ; in an active arboreal life a 

 rapid and effective adjustment becomes vital for safety ; while the 

 need for close examination of objects manipulated by the hands 

 becomes of crucial importance in the activities of the higher Primates 

 and man. 



Few vertebrate species are entirely without accommodation, and 

 to these vision is invariably of little biological importance. Such a 

 mechanism is lacking in the extant representatives of the primitive 

 groups of Fishes — Chondrosteans, Dipnoans ^ and the coelacanth ; the 

 function of the campanular muscle of Holosteans has not been 

 explored. Among Amphibians and Sauropsidans an accommodative 

 mechanism is present except perhaps in Sphenodon. Owing to their 

 nocturnal habits, accommodation is lacking or exceedingly feeble 

 in primitive Mammals. It is absent in Monotremes, and although a 

 ciliary muscle is present in Marsupials, no accommodation has been 

 demonstrated in any species of this group. In the lower Placentals 

 accommodative activity is similarly lacking for the ciliary muscle is 

 vestigial if, indeed, it is present (except in squirrels) ; even Ungulates 

 such as the horse, sheep and pig have no demonstrable dynamic 

 accommodation, and apart from the feeble accommodation of squirrels 

 and Carnivores, an effective range is found among Mammals only in 

 the otter and in the Primates, particularly in man. 



Within the vertebrate phylum accommodation is achieved by a 

 great variety of devices ; it would appear as if at one time or another 

 in the various species every conceivable means of adjusting the 

 dioptric system of the eye to various distances had been attempted. 

 These varying expedients may be classified into two types — static 

 devices whereby optical elasticity is achieved by structural peculiarities; 

 and djmamic devices depending upon an active alteration in the 

 dioptric system brought about by muscular activity. 



Static Devices 



In the first place, it is to be remembered that a swmU eye to a 

 large extent obviates the need for an active accommodative mechanism. 

 A small lens with a short focal length has a greater depth of focus than 



' The eye of Neoceratodus is unexplored from this point of view. 



