CHAP. XII.] AND PERCEPTION. 191 



by the Eye of the primary qualities of the external world, 

 as well as an appreciation of the distance and direction of 

 sounds on the occasion of the first exercise of the Ear ; 

 (2) That chickens instinctively bring into action Muscles 

 that were never so exercised before, and perform a series 

 of delicately Adjusted Movements ending in the accom- 

 plishment of a definite act independent of any antece- 

 dent experience, and, therefore, of any ' conception ' of 

 such act ; (3) That " in the more important concerns of 

 their lives animals are guided by Knowledge which they 

 individually have not gathered from experience." 



Other facts illustrative of these truths will be found 

 recorded in the chapter on * Instinct.' 



General Statement of Results. Our brief survey of 

 the structure of Nervous Systems, in passing from their 

 simplest forms to those possessed by Birds, has shown 

 that they tend to become more and more complex as 

 animals rise in the scale of organization. We have also 

 seen reason for believing that the Mental and Motor 

 phenomena, of which such organisms are capable, show 

 a similar tendency to increase in complexity. 



The increase in structural complexity is brought about 

 by the growth and development of nerve-tissues in the indi- 

 vidual under the stimulus of sensory ' experience ' ; aided 

 by the continuation, through the principle of heredity, 

 of such new developments in succeeding individuals. By 

 the more or less universal repetition of such processes 

 along different lines of development, slow structural pro- 

 gressions have been achieved; and with them have arisen 

 corresponding developments of function in the direction of 

 Mental and Motor Phenomena. Just as, in the individual, 

 the appearance of new structure and the occasional mani- 

 festations of new function have been coeval ; so the * in- 



