18 OUTLINES OF EVOLUTIONARY BIOLOGY 



the contractile vacuole is probably to be regarded as the special 

 excretory organ of the animal. 



In all but the lowest animals there is a more or less specialized 

 nervous system, whose function it is to place the different parts 

 of the body in communication with one another and, through 

 the mediation of the sense organs, with the external world or 

 environment. The action of this system depends primarily 

 upon one of the fundamental properties of living protoplasm, 

 the power of responding to stimuli by some definite change 

 in its own condition. The stimulus is, in the first instance, 

 supplied by some factor of the environment, such as light, heat, 

 electricity or mechanical impact. It may appear to originate in 

 the central nervous system itself, but this is probably secondary. 

 It has the effect of liberating stored energy in those parts of 

 the organism which are sensitive to that particular stimulus, 

 in somewhat the same way that the stimulus of heat may 

 have the effect of liberating the stored energy in a charge of 

 gunpowder. The living molecule has, indeed, actually been 

 described as explosive. In both cases potential energy is 

 converted into kinetic energy, and the effect which is produced 

 may be out of all proportion to the amount of energy represented 

 by the liberating stimulus itself. 



In the higher animals, then, the stimulus received from the 

 external environment, whatever its nature may be, acts 

 primarily upon some special sense organ or receptor, whence it 

 is transmitted along highly specialized tracts of tissue, the 

 nerves, to some part of the central nervous system, where it 

 usually gives rise to what we call a sensation. The central 

 nervous system, again, not only has the power of receiving stimuli 

 through afferent or sensory nerves, but also of sending stimuli 

 through efferent nerves to the various organs of the body, 

 whereby their functions are controlled and regulated. The con- 

 traction of muscles and the secretion of glands are all con- 

 trolled in this manner, and the entire working of the body is 

 co-ordinated by the action of the nervous system. 



In the Amoeba, however, we see no trace of a special nervous 

 system, nor of sense organs, but nevertheless the organism is 

 certainly capable of receiving and responding to stimuli ; in 

 other words it is irritable. Thus the protoplasmic body responds 

 by contraction to the stimuli of mechanical impact, heat, light, 

 electricity and chemical reagents, and, as we have already seen, 



