CHAPTER II. 



PROTOPLASM: ITS MORPHOLOGY AND PHYSIOLOGY. 



13. Life. Life may be thought of in two somewhat dis- 

 tinct ways. It may be considered, first, merely as an expres- 

 sion for all the various activities of the organism, the sum 

 of all the phenomena of its existence ; or, second, as a force or 

 form of energy from which the special modes of activity, as 

 feeding, growth,' motion, and thinking, arise. The latter is 

 the more common use of the term, and yet the former is the 

 only use of it which can be completely justified. Much of the 

 activity of living things may be explained by reference to the 

 ordinary physical and chemical laws. At least we know that 

 these latter processes underlie all the actions which we call 

 vital. It is indeed a question whether all vital phenomena are 

 not finally to be explained by means of them, without the need 

 of assuming any special vital principle or force. There seems, 

 however, a growing disposition among scientists to admit that 

 the action of chemical force does not suffice to explain all the 

 phenomena of the living animal. Whether this is true or not, 

 it is often convenient to speak of " vital force " as if it were a 

 cause embracing more than is usually included in the knoiun 

 chemical and physical actions. 



14. The Relation of Protoplasm to Life. Whatever life 

 may be, in the last analysis, we never observe its manifestations 

 except in connection with a substance called protoplasm, which 

 is found both in plants and animals. Protoplasm does not con- 

 tain any chemical elements which are not found in other than 

 living materials. Notwithstanding this fact, protoplasm is 

 different from any other known substance. It is more com- 

 plex and more highly organized, as to its machinery, than any 

 other chemical or physical compound whatsoever. Protoplasm 

 has the power of growing by taking up and changing other 



