THE PHYSICAL BASIS OF LIFE 275 



substance of organism, is practically the same in plants a no 

 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 0] Species and Spencer's First Principles. 



