son of human development with that of other animals, but 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 compelled 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 receiving and responding to stimuli: this is one of the fundamental 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 sensitivity
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.)
(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 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 locomotion, 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 protoplasmic prominence into which the sperm is received (Figs. 4, 5, E C). It is an interesting fact
