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 
which 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 development, not in kind, from those we see in bodies of 
higher organisms. 
We can watch the movements in this amceba, 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 amceba 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 amceba is receptive and assimilative, that it is 
respiratory, taking in oxygen and giving off carbonic dioxide, 
and that it is also secretory. If the amceba 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 amceba 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 amceba is found the germ of 
all the activities of the higher animals. 
