CoGiTic Evolution of Man 613 



themselves in such stereoenergetic Hnes through the responsive 

 cells that these elongate or grow more in one direction line 

 than in other directions. This line of greatest growth or of 

 energizing activity becomes the response line. 



But a considerably more complex condition is introduced 

 with the ganglion cell, and is witnessed in such movements as 

 those of the leeches above described. As Loeb has insisted 

 however, complexity need not cause us to regard a condition 

 as one of insoluble mystery. One fundamental requirement 

 for a satisfactory explanation, as Romanes and more recently 

 Loeb have sIiowti, is the gradual development of the physico- 

 chemical relation that has been called associative memory. 

 This has too often in the past been regarded as a semi-meta- 

 physical abstraction, without material basis. 



To trace its rise as a condition that is bound up with the 

 ganglion cell, and with cogitic acts, we must try to learn what 

 intermediate states arose in transition from the higher uni- 

 cellular and the simpler multicellular organisms devoid of 

 ganglia, like the Spongida and the Mesozoa, to the simpler 

 gangliate types. In these, as in still lower animals and in most 

 plants, the tendency for separate energizing stimuli to combine 

 their molecular actions and reactions into a resultant molecu- 

 lar combination becomes so mechanical and routine in relation 

 that even in absence of the stimulus a definite motion may occur. 



Thus, to cite a single case from plant life, the so-called sleep 

 or nyctitropic state in which leaflets fold together is primarily 

 due to withdrawal of light, but at the same time apogeotropism 

 or diageotropism is acting on the sensitive pulvini or cushions 

 to keep each leaflet in definite position in relation to gravity. 

 The actual position assumed then during night is a resultant 

 of these two actions, possibly even of a third or chemotropic. 

 On the approach of morning light, the leaflets again expand 

 fully. But if a plant be kept in darkness for two or more days 

 it is found that the rhythmic periodicity of plant-movement is 

 such that about the time of normal ** awaking" tlie leaflets will 

 expand more or less perfectly even in absence of light. Here 

 we would consider is a beginning indication of associative mem- 



