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 cf biology, were for the first time brought 
directly to the attention of the naturalist, about the year 
1860—that epoch-making time when appeared Darwin’s 
Origin of Species and Spencer’s First Principles. 
