234 Causes and Course of Organic Evolution 



The gradual localization, in a few cells, of substances that 

 undergo such rhytlunic transformations, under the action of 

 even slight environal change, would constitute, we believe, 

 one initial cause for origin of nerve cells. 



Another initial cause for such origin would be the cumu- 

 lative association of two or more distinct stimuli in cells of a 

 localized area, and the subsequent gradual separation of sister 

 cells from these, or the linking up of these mth separate cells or 

 groups of cells, that would act as the response-discharging cells. 



Still another cause that has been recognized and discussed 

 by Loeb, Thorndike, and others is association and associative 

 memory. In this case, as a result of the utilization and cumu- 

 lation of several simple sense impressions, a resultant response 

 is started; or even several resultants, from groups of simple 

 stimuli, may be compounded, and may then give rise to a 

 satisfied response. Repetitions of such actions, at first rather 

 imperfectly, become more perfect and easy, as a definite asso- 

 ciation and succession of colloid molecular combinations is 

 effected in rhythmic relation. Such doubtless are in turn 

 connected with definite molecular disintegrations and com- 

 binations in definite constituents of the nerve cells. So, when 

 even a few of the primary stimuli — that wdth others for the 

 moment may be absent but which ordinarily make up the 

 complete stimulus — become appropriately combined, these 

 tend to lead up to, and work out, the ultimate resultant,' owing 

 to tubes of energy-flow along lines of least resistance. 



So chronic or rhythmic vibrational excitation in a cell or 

 group of cells, cumulative association of several distinct result- 

 ants in definite nerve cells, and passage of a compounded 

 resultant response to other cells detached from or linked up 

 with these, also associative flows of tubes of energy along 

 connected molecular lines, and impression of these more and 

 more deeply on the molecular organization, seem all to be 

 ])rimitive causes for origin of the highly complex nerve-cell. 



With such increasing complexity in the cumulation of dis- 

 tinct stimuH, it might be expected that certain probably rec- 

 ognizable bodies should characterize the nerve cells; should 



