380 COMPARATIVE ZOOLOGY OF CHORD ATES 



ear, and nose, can perceive objects at a distance : the distance- 

 receptors. Responses to stimuli which touch the animal 

 usually (when successful) abolish the stimulus which evoked 

 them. So the flea tickling the dog on its skin evokes the 

 scratch which incapacitates the flea from tickling any more. 

 Such a response is consummatory. If, however, an animal 

 sees some of its food at a distance, the response which it makes 

 to start with does not abolish the stimulus. It sets its limbs 

 in motion towards the food ; this is an anticipatory response, 

 and the consummation is not complete until the food has been 

 reached and eaten. Until this time, the food occupies the 

 attention of the animal. 



In the reptiles, there are three sheets of superficial grey 

 matter in each cerebral hemisphere. The median sheet is 

 the hippocampal and the lateral sheet the pyriform cortex. 

 Both these regions are predominantly concerned with impulses 

 coming from the nose ; they are not really " impartial " 

 arbitrators of behaviour. That the cerebral hemispheres 

 should in early stages of evolution be largely under the influence 

 of olfactory sensations follows from the proximity of the 

 olfactory lobes, and from the fact that at these stages the 

 vertebrates had recently emerged from life in water to dry 

 land, for the nose is a more highly developed and efficient 

 organ in air than in water. Being at the most anterior end of 

 the brain, it naturally took time in evolution before fibres 

 from all the correlation-centres farther back in the central 

 nervous system reached them. Part of the middle sheet in 

 the cerebral hemispheres of the reptile appears to be the fore- 

 runner of the true cerebral cortex, which reaches such a high 

 development in the mammals. The hippocampal and pyri- 

 form cortex are called archipallium, to distinguish them from 

 this neopallium in which olfactory impulses do not predominate. 



In the birds the cerebral cortex is less well developed than 

 in the reptiles, and the corpus striatum with the attendant 

 highly instinctive type of behaviour is specialised instead. 



In the mammals, the cerebral cortex is developed out of 

 proportion to the rest of the brain. In the higher mammals 

 (but not in Monotremes or Marsupials) a special commissure 



