210 Causes and Course of Organic Evolution 



been made to the proenvironal capacity of Chlamydomonas (p. 

 166). We may therefore study unicellular animal responses. 



In nearly all such cases it is observed that not the entire 

 organism, but some polar center or line of stimulation, is the 

 receptive area, and that, when one or more stimuli act on 

 this, a period of excitation elapses during which the living 

 molecules are plotting a proenvironal pathway, that eventually 

 is pursued. Further, from the experiments of Jensen, Jen- 

 nings, and others, we learn that if the organism is free swim- 

 ming and is exposed to a single new stimulus, even though 

 this be favorable or satisfying, it may during the excitation 

 period retreat slightly. From the experimental e\'idence at 

 hand we would interpret this as being to permit of time for 

 action of the stimulant energy on the molecules of the organ- 

 ism, and so for determining whether favorable or unfavorable 

 response to the stimulant energy should be made. Or in 

 other words such constitutes what the plant physiologists 

 term the perception phase in the latent or excitation period. 



Abundant evidence in favorvof proenvironal response, when 

 a single stimulus acts, is to be found in many works, and some 

 of these are synopsized by Davenport Gl), Jennings (^^), and 

 J. Loeb {59). But results for two or more stimuli are often 

 as yet uncertain or even contradictory. Thus Jennings {1^2: 

 92-98) considers that, where two stimuli act, any one of four 

 possibilities are sho^A*n for Parainoecium^ namely either: (1) 

 that if a stimulus a is acting and another stimulus b be applied 

 it will react to b in the usual way; or (2) it will continue to 

 react to a; or (3) it will form a compromise "between the usual 

 reactions to the two agents"; or (4) it will react in a new way, 

 different from the usual reactions to either a or b. But in 

 several of these cases many experiments were not made to test 

 whether a definite optimum, as well as maximum and mini- 

 mum, for each kind of stimulus existed. This however is 

 necessary for every physiological action. 



But one could well exj^lain 1, 2, and 4, that seem uncertain, 

 as being truly resultant effects. Thus, if in 1 stimulus a be 

 rather weak and applied at a different angle from b that is 



