296 THE EVOLUTION OF ANIMAL BEHAVIOUR 
a sentience which is as yet ineffective since the process of 
conscious coalescence has not begun, or has not been carried 
far enough. 
In foregoing chapters we have constantly held the problems 
of evolution in view, and in special sections directed attention 
to them. But the subject is so central to modern thought and 
discussion, that some further consideration of certain aspects 
of the evolutionary process and products will fitly serve to 
bring our inquiry to a conclusion. 
We must accept, as a datum from the physiological point 
of view, the fact that protoplasm does respond to stimuli,—that 
it possesses the fundamental property of irritability. It isa 
substance that is in a state of unstable equilibrium. Its ten- 
dency to pass to a condition of more stable equilibrium is that 
in and through which organic behaviour in its very simplest 
expression is possible. And this, with progressive complication, 
runs through the whole gamut of animal behaviour, and 
eventually passes over into the sphere of consciousness. ‘ The 
tendency to equilibrium,” writes Dr. Stout,* “is the physio- 
logical correlate of what on the psychical side we call conation, 
—the striving aspect of consciousness.” But, protoplasm at the 
outset—or as near the outset as we can get—is, in technical 
phrase, differentially responsive. The nature of the stimulus 
and the nature of the conditions decide what the nature of 
the response shall be. And even in that jelly-like speck of 
living matter, the Ameba, the responses conspire to a biological 
end. If they did not so conspire, we should not have the 
phenomena of life. The mere act of living, building up from 
food-stuff and oxygen an unstable substance which “ ex- 
plodes” and contracts under stimulation, implies that the 
processes which thus conspire are related in such a manner as 
to fulfil and secure their end. In higher unicellular animals, 
such as the Paramecium, the relations are less simple; but in 
them the continuance of that sum of organic behaviour which 
we call life, is secured only on the condition that these less 
simple relations are duly preserved, and that the vital processes 
* “Manual of Psychology,” p. 132. 
