260 BIOLOGY AND ITS MAKERS 



of the microscope. This little animal consists almost entirely 

 of a lump of living jelly. Within the living substance of 

 \\hich its body is composed all the vital activities character- 

 istic of higher animals are going on, but they are manifested 

 in simpler form. These manifestations differ only in degree 

 of developm.ent, not in kind, from those we see in bodies of 

 higher organisms. 



We can watch the movements in this amoeba, deter- 

 mine at first hand its inherent qualities, and then draw up 

 a sort of catalogue of its vital properties. We notice an 

 almost continual flux of the viscid substance, by means of 

 which it is able to alter its form and to change its position. 

 This quality is called that of contractility. In its essential 

 nature it is like the protoplasmic movement that takes place 

 in a contracting muscle. We find also that the substance 

 of the amoeba responds to stimulations — such as touching 

 it with a bristle, or heating it, or sending through it a light 

 electric shock. This response is quite independent of the 

 contractility, and by physiologists is designated the property 

 of being irritable. 



By further observations one may determine that the sub- 

 stance of the ama-ba is receptive and assimilative, that it is 

 respirator}^ taking in oxygen and giving off carbonic dioxide, 

 and that it is also secretory. If the amoeba be watched 

 long enough, it may be seen to undergo division, thus produc- 

 ing another individual of its kind. We say, therefore, that it 

 exhibits the power of reproduction. All these properties 

 manifested in close association in the amoeba are exhibited 

 in the bodies of higher organisms in a greater degree of 

 perfection, and also in separation, particular organs often 

 being set apart for the performance of one of these par- 

 ticular functions. We should, however, bear in mind that 

 in the simple protoplasm of the amoeba is found the germ of 

 all the activities of the higher animals. 



