4 BRAIN MECHANISMS AND LEARNING 



dominant hemisphere, with the capabihties of Pavlov's second signal 

 system for symbolization and communication by means of spoken and 

 written language. 



Further testimony for the evolution of neurological function in these 

 terms was provided by Jackson's view of dissolution, or reversal of the 

 phylogenetic process when clinical impairment proceeded from highest 

 through middle to lowest levels during neurological disease in man 

 (Jackson, 1958). Jackson specified that the resulting deficit was usually 

 accompanied by some release of lower activity, normally subjugated to 

 higher control. This latter feature was elaborated also in the Freudian 

 system, in which conflicting interests of the different levels were emphasized 

 as a source of psychic disturbance. 



In much the same way that increased complexity and specialization 

 appeared as the ladder of nature was ascended by the earlier classification- 

 ists, more and more elaborate functions came to view as one climbed 

 cephalically up the successive levels of the central nervous system. In its 

 progressive enccphalization, the brain came to resemble the earth itself, not 

 simply in its globular form but in consisting as well of a series of strata laid 

 down like those of geology, one upon the other, in evolutionary time. 

 Each neural accretion was associated with a characteristic increment of 

 function and, following Jackson, a dis-solutionary school of neurophysio- 

 logy developed, in which enccphalization was reversed by operative 

 transection and evolution traced backward by observing residual capaci- 

 ties diminish in the increasingly truncated, decorticate, decerebrate and 

 spinal preparations. 



Probably because such views arc still so contemporary, little attention 

 has been given to exploring the role of Darwin, and the interest in evolu- 

 tion excited by his work, in establishing these concepts of neural organiza- 

 tion and function. The views of Hughlings Jackson (1958), which might be 

 presumed to be the most directly Darwinian, were, on the contrary, 

 derived chiefly from Thomas Laycock, with wht~)m Jackson began his 

 career in York, and from Herbert Spencer, whom he admired greatly. 

 Both Laycock and Spencer had applied evolutionary principles to con- 

 cepts of the organization and function of the brain independently of and 

 preceding Darwin. In his Mind and Brain, first published in 1859, Laycock 

 wrote: 'As wc ascend the scale, the difterentiation of tissue takes place and 

 instincts of plants or animals appear. As we ascend still higher in animal 

 life, the instincts gradually lose their unknowing character and the mental 

 facidties emerge with their appropriate organic basis in the encephalon. 

 Finally, with the highest evolution, we find man evincing in art and 



