236 THE EYE IN EVOLUTION 



earth for more than 100 miUion years perished, apart from a few unimportant 

 exceptions — the very primitive Chelonians (tortoises and turtles), the almost 

 extinct Rhynchocephalian, Sphenodon, of lineage almost as remote ; and the 

 more modern groups, lizards and snakes and crocodiles. 



The handicap of cold-bloodedness limited these surviving Reptiles to the 

 warmer parts of the earth. In the even temperature of the sea the Teleosteans 

 could flourish without hindrance ; to populate the cooling earth homeostasis 

 had to be achieved ; this was eventually acquired by Birds and Mammals, the 

 former assuming an insulating coat of feathers, the latter usually of hair in 

 place of the scales characteristic of Reptiles. Of the two the Mammals 

 claim the more primitive descent, stemming from the Therapsidans, mammalian- 

 like Reptiles which flourished in Permian and early Triassic times. During the 

 latter period it would seem that Mammals made their appearance as small 

 mouse-sized creatures, but throughovit the entire Mesozoic era they appear to 

 have been sparse, leaving few fossil remains ; it was not until the end of the 

 Cretaceous period, 75 million years ago, when the great carnivorous Reptiles 

 finally died off that these retiring, inconspicuous creatures, probably nocturnal 

 or arboreal in habit, were able to take the leading place in evolvitionary progress. 

 This they have done to such good purpose that they have adapted themselves 

 to and become completely predominant in almost every environment on land, 

 some of them even returning to the water wherein their lately acquired superiority 

 afforded them a relatively easy existence (whales, seals, Sirenians) while others 

 (bats) have invaded the air. 



Parallel with the Therapsida stands the other reptilian group of Sauropsida, 

 of which lizards and snakes are a direct off-shoot ; from it was derived the 

 Archosauria, a group characterized by a limb-and-girdle structure enabling them 

 to run semi-erect upon their hind legs with a bipedal gait. The only members 

 of this stock which have survived are the crocodiles and their relatives the 

 alligators ; but, particularly in their most spectacular forms, the Dinosaurs, 

 some of them as heavy as 40 or 50 tons, they constituted the dominant terrestrial 

 type during the latter half of the Mesozoic era. From these are descended 

 modern Birds which show innumerable reptilian features. 



Curiously it was from the most primitive type of placental Mammal, the 

 Insectivores, that the Primates and Man evolved, and in the evolution of these 

 the great advance has been associated with the brain. This was achieved in a 

 peculiar way. A small and unimportant group became adapted to arboreal life, 

 thus developing their cortical capacity by the coordination of the eye and hand ; 

 thereafter, descending from the trees and freeing their hands by becoming 

 bipedal, they passed the critical point at which physical dexterity could 

 combine with conceptual thought and the faculty of speech, and thus a new 

 method of evolution became possible based on the transmission of cultural 

 experience. At this stage the potentialities of vision are measured not by 

 the optical and structural excellence of the receptive end-organ, but by the 

 apperceptive capacity of the mind. In this way, just as the Mammals defeated 

 the lower Vertebrates on land, leaving the water to the Teleosts and the air to 

 the Birds, so the Placentals eliminated the Monotremes and Marsupials wherever 

 they came in contact with them, the Carnivora dominated the lower Placentals, 

 the monkeys the Prosimians, and finally Man triumjihed over all the others. 



From the anatomical point of view — and certainly from the aspect of the 

 structure of the eye — these six classes of the Vertebrates, neglecting the Cyclo- 

 stomes, can conveniently be reduced to three great groups as suggested by 

 Huxley : 



