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THE POPULAR SCIENCE MONTHLY 



son of human development with that of other animals, hut the great 

 principle of the oneness of life, as respects its fundamental processes, has 

 never yet failed to hold true and will not fail us here. In the study of 

 the psychical processes of organisms other than ourselves we are com- 

 pelled to rely upon a study of their activities, their reactions to stimuli, 

 since we can not approach the subject in any other way. The reactions 

 and behavior of organisms under normal and experimental conditions 

 give the only insight which we can get into their psychical processes — 

 and this applies to men no less than to protozoa. 



1. Sensitivity. — The most fundamental phenomenon in the behavior 

 of organisms is irritability or sensitivity, which is the capacity of re- 

 ceiving and responding to stimuli : this is one of the fundamental prop- 

 erties of all protoplasm. But living matter is not equally sensitive 

 to all stimuli, nor to all strengths of the same stimulus. Many of the 

 simplest unicellular plants and animals show that they are differentially 

 sensitive; they often move toward weak light and away from strong 

 light, away from extremes of heat and cold, into certain chemical sub- 

 stances and away from others — in short, all organisms, even the simplest, 

 may respond differently to different kinds of stimuli or to different 

 degrees of the same stimulus. This is what is known as differential sen- 



Fig. 17. Distribution of Bacteria in the Spectrum. The largest group is in 

 the ultra-red at the left ; the next largest group is in the yellow-orange close to the 

 line D. (From Jennings, after Engelmann.) 



sitivity (Figs. 17, 18, 19.) On the other hand, many organisms respond 

 in the same way to different stimuli and this may be taken to indicate 

 generally that they are not differentially sensitive to such stimuli ; it is 

 not to be concluded that because organisms respond differently to certain 

 stimuli they are therefore capable of distinguishing between all kinds 

 of stimuli, for this is certainly not true. Even in adult men the capacity 

 of distinguishing between different kinds of stimuli is far from perfect. 

 Egg cells and spermatozoa show this property of sensitivity. The 

 egg is generally incapable of locomotion, and since the results of stimu- 

 lation must usually be detected by movements it is not easy to determine 

 to what extent the egg is sensitive ; but though the egg lacks the power 

 of locomotion, it possesses in a marked degree the power of intra-cellular 

 movement of the cell contents. When a spermatozoon comes into con- 

 tact with the surface of the egg the cortical protoplasm of the egg flows 

 toward that point and may form a cone or protoplasmic prominence into 

 which the sperm is received (Figs. 4, 5, E C). It is an interesting fact 



