10 PRINCIPLES OF PHYSIOLOGY 



they pass through a number of stages in their 

 existence, they reproduce their kind, and they 

 die. It may not always be easy to observe 

 some of these phenomena in the lower forms, 

 but we find that their bodies are composed of 

 matter that possesses certain properties, and 

 we characterize such matter as being alive. 



3. One of the lessons of scientific investiga- 

 tion is that in the study of phenomena we find 

 transition a series of changes, and the gradual 

 passage of one state into another while a 

 superficial examination may appear to establish 

 clear lines of division between different depart- 

 ments of knowledge. Thus we distinguish 

 between that which we say is dead matter, 

 and that which we consider to be alive. 

 More careful examination, however, shows 

 that certain properties may be the same, or 

 similar, in both dead and living matter. Thus 

 a crystal, which we regard as dead, grows 

 and increases in size in accordance with 

 physical laws. Living matter also grows and 

 increases in size, but by a different process 

 from that of a crystal. So that mere increase 

 in size, in certain conditions, may characterize 



