Pacts and Factors of Development 37 



stimuli, since we can not approach the subject in any other way. 

 The reactions and behavior of organisms under normal and ex- 

 perimental conditions give the only insight which we can get into 

 their psychical processes ; and this applies to men no less than to 

 protozoa. 



i. Sensitivity. The most fundamental phenomenon in the be- 

 havior of organisms is irritability or sensitivity, which is the abil- 

 ity of receiving and responding to stimuli : this is one of the fun- 

 damental properties 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 substances 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 sensitiv- 

 ity (Figs. 15-19). 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 because organisms respond differently to 

 certain stimuli that 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. 



Sensitivity of Germ Cells. Egg cells and spermatozoa show 

 this property of sensitivity. The egg is generally incapable of lo- 

 comotion, and since the results of stimulation 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 locomo- 

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

 movement of the cell contents. When a spermatozoon comes 

 into contact with the surface of the egg the cortical protoplasm 

 of the egg flows toward that point and may form a cone or pro- 

 toplasmic prominence into which the sperm is received (Fig. 



