THE PHYSICAL BASIS OF LIFE 275 



substance of organism, is practically the same in plants and 

 in animals. Let us picture to ourselves the consequences of 

 the acceptance of this idea. Now for the first time physiol- 

 ogists began to have their attention directed to the actually 

 living substance; now for the first time they saw clearly 

 that all future progress was to be made by studying this living 

 substance — the seat of vital activity. This was the beginning 

 of modern biology. 



Protoplasm is the particular object of study for the biol- 

 ogist. To observe its properties, to determine how it be- 

 haves under different conditions, how it responds to stimuli 

 and natural agencies, to discover the relation of the internal 

 changes to the outside agencies: these, which constitute the 

 fundamental ideas of biology, were for the first time brought 

 directly to the attention of the naturalist, about the year 

 i860 — that epoch-making time when appeared Darwin's 

 Origin oj Species and Spencer's First Principles. 



