TRANSACTIONS OF SECTION D. 427 



has in great measure lost precisely those features which are essential if it is to 

 develop into the reptilian or mammalian condition. 



Perhaps both Amphibia and Roptilia derived directly from Stegocephalia. 



Recent research, especially in regard to fossils from the Permian of Texas, 

 has brought to light Stegocephalians so closely resembling reptiles, and reptilian 

 remains so Stegocephalian, that there can no longer be any question of the 

 genetic relationship between the two groups. 



Although many biologists are apt to lay emphasis upon the aberrant and 

 specialised features of the Dipnoi, and look to the Crossopterygii for the deriva- 

 tion of the Stegocephalia, the features of the brain in the lung fishes so 

 definitely foreshadow the conditions seen in the Reptilia that it is difficult to 

 believe that the Dipnoi can be far removed from the direct path leading to the 

 Amniota. 



At the same time, the Dipnoan brain, in its general plan, though not in its 

 histological detail of its cortex, is essentially Amphibian. This fact, taken in 

 conjunction with the palaeontological evidence, suggests that the Stegocephalian 

 brain may have bridged the gap between those of the Dipnoi and the Reptilia. 



It is unfortunate that we know nothing of the form of the brain in the 

 Cynodonts, for the transformation of its cortex must have played a leading part 

 in the evolution of the Mammalia. 



Broom tells us that the South African Therkidonts, from which mammals 

 were derived, became distinguished from their American allies by the develop- 

 ment of powerful limbs, and that ' it was the lengthened limb that gave the 

 start to the mammals.' ' When the Therapsidan took to walking with its feet 

 underneath and its body off the ground it first became possible for it to become 

 a warm-blooded animal. All the characters that distinguish a mammal from a 

 reptile are the result of increased activity — the soft flexible skin with hair, the 

 more freely movable jaws, the perfect four-chambered heart, and the warm 

 blood.' 



Broom confesses his inability to explain how this fateful lengthening of the 

 limbs was caused. 



The realisation of the changes which took place in the brain in the transition 

 from reptiles to mammals seems to suggest an explanation of this and the acquisi- 

 tion of many other mammalian features. 



The development of a definite neopallium (the cerebral cortex sensu stricto), 

 the lengthening of the limbs, the increased activity, the freeing of the skin 

 of its mail-like coat of scales and conversion of it into a highly developed tactile 

 organ — all these events occurred at about the same time, and had a reciprocal 

 influence one upon the other. 



By the time the Reptilia were evolved the cerebral hemisphere had reached 

 a stage of development which opened up vast possibilities of new developments. 

 Though the cerebral cortex was still mainly olfactory in function, tactile, gusta- 

 tory, visual, and perhaps auditory impulses were able to make their entry into 

 it ; but it exercised little direct control over the movements of the body, which 

 were still regulated by the midbrain. 



The possession of this potential receptive organ in the cortex for receiving 

 tactile impressions and bringing them into relation with impressions from the 

 other sense-organs gave an added importance to the tactile sensibility of the 

 ridges of skin that intervened between the scales of the Hypotherian. More- 

 over, more precise movements of the limbs became possible, because more exact 

 information was being provided of the positions of the limbs by these tactile 

 impressions. 



The enhanced importance of the skin as a tactile organ led to the atrophy of 

 the scales, perhaps by a process of natural selection ; and the greater perfection 

 of the tactile sensibility of the skin on the one hand, and of its receiving and 

 recording apparatus in the cortex on the other, reacted mutually one upon the 

 other and gave birth to the neopallium. It is not without significance that 

 from its earliest appearance the neopallium performed the function of regulating 

 'skilled' movements of the whole body, i.e., such actions as are possible only 

 when there is a highly developed tactile information-bureau to render nicely 

 adjusted movements possible. Moreover, quickness and increased activity are 

 made possible by the neopallium, because it was put into direct connection ab 

 initio with all the motor nuclei in the whole central nervous system by the 



