226 COMPARATIVE ZOOLOGY 



and above the known forces of the laboratory. 68 If the 

 vital machine is given, it works by physical forces ; but 

 to produce it and keep it in order needs, so far as we 

 now know, more than mere physical force. To this 

 controlling power we may apply the name vitality. 



Life is exhibited only under certain conditions. One 

 condition is the presence of a physical basis called proto- 

 plasm. This substance is found in all living bodies, 

 and, so far as we know, is similar in all a viscid, 

 transparent, homogeneous, or minutely granular, albu- 

 minoid matter. Life is inseparable from this proto- 

 plasm ; but it is dormant unless excited by some 

 external stimulants, such as heat, light, electricity, food, 

 water, and oxygen. Thus, a certain temperature is 

 essential to growth and motion; taste is induced by 

 chemical action, and sight by luminous vibrations. 



The essential manifestations of animal life may be 

 reduced to four : contractility ; irritability, or the power 

 of receiving and transmitting impressions ; the power of 

 assimilating food ; the power of reproduction. All these 

 powers are possessed by protoplasm, and so by all ani- 

 mals : all move, feel, grow, and multiply. But some of 

 the lowest forms are without any other trace of organs 

 than is found in a simple cell ; they seem to be almost as 

 homogeneous and structureless as a drop of jelly. They 

 could not be more simple. They are devoid of muscles, 

 nerves, and stomach ; yet they have all the fundamental 

 attributes of life, moving, feeling, eating, and propa- 

 gating their kind. The animal series, therefore, begins 

 with forms that feel without nerves, move without mus- 

 cles, and digest without a stomach, protoplasm itself 

 having all these properties : in other words, life is the 

 cause of organization, not the result of it. Animals do 

 not live because they are organized, but are organized 

 because they are alive. 



