158 



Mr. G. A. Watson. 



[July 28 , 



child the combined fourth and fifth layers are little over the depth of those 

 layers in the Mole. In the normal human adult these combined layei's are 

 only a little over half as deep again as in the Mole (T56 to 1). 



The increase in depth of the human cortex cerebri as compared with 

 that of the Mole is therefore very largely due to increase in the supra- 

 granular layer. 



Conclusions as to tloc Functional Significance of the Supra-Granular and Infra- 

 Granular Cortical Layers. 



Bolton's views as to the functional value of these layers have already 

 been briefly stated (p. 151). The following conclusions, which apply only 

 to mammals, form a complement of them from the point of view of the 

 Insectivora and of the lower mammals belonging to various other natural 

 orders so far examined. 



1. The infra-granular layer (IV and Y) (omitting the constituent cells 

 which possess motor or analogous functions), which is relatively so fully 

 developed at birth, is concerned especially with the associations necessary 

 for the performance of the instinctive activities, that is, all those which are 

 innate and require no experience or education. These involve many complex 

 actions, such as the seeking appropriate shelter and protection {e.g., tunnelling 

 of Mole), the hunting for food — each after his own kind — and the quest 

 of the opposite sex. Such more or less stereotyped activities may show 

 signs of improvement, firstly as the result of the perfection by use of an 

 inherited mechanism, and secondly as the result of the intermingling of 

 activities for which it is concluded that the supra-granular layer is 

 responsible. In the latter case, however, the activities would merge into 

 those which are more properly described as habitual intelligent or into the 

 class of incomplete instincts (Lloyd Morgan), or mixed instincts (Eomanes). 



2. The supra-granular (pyramidal) layer subserves the higher associations., 

 the capacity for which is shown by the educability of the animal. It has 

 therefore to do with all those activities which it is obvious that the animal 

 has acquired by individual experience, and with all the possible modifications 

 of behaviour which may arise in relation to some novel situation, hence 

 with what is usually described as indicating intelligent as apart from 

 instinctive acts. 



In practical animal behaviour the two sets of processes are probably more 

 or less constantly interwoven, the higher activities (supra-granular layer) 

 coming to the aid of the lower as far as the capability of the animal allows. 

 In the case of the lower mammals {e.g., Insectivora) the limits of this 

 capability are comparatively soon reached, and, correspondingly, these 



