324 THE EVOLUTION OF ANIMAL BEHAVIOUR 
cannot be carried far. Theories without the power of dis- 
putation would be a poor solace in leisure moments. 
One more point may be noticed with regard to the psycho- 
logical aspect of the evolution of behaviour—the reciprocal 
action of intelligence. It is the intelligence of others that 
introduces so much variety and complexity into the environ- 
ment. Hunters and hunted, combatants, rivals, mate and 
mate, enemies or companions in their varied aspects, introduce 
through their intelligence complications which only intelligence 
can meet. And, as intelligence begets intelligence, so do 
emotional attitudes beget answering emotional states. Psycho- 
logical evolution translated into practical behaviour gives rise 
to situations of reciprocal complexity. ‘This point of view is, 
however, so familiar, that nothing need be said in its further 
elucidation. The behaviour of any given animal does not 
stand alone, but is closely related with the behaviour of others. 
Among social animals the relationships are peculiarly close, 
and it is among them that the psychological aspect of behaviour 
reaches its highest expression. 
IV.—ContTInuIty In EvoLurion 
Under the head of organic behaviour, in the widest ac- 
ceptation of the term, fall the whole of physiology, the whole 
of embryological development, nay, more, the whole of organic 
evolution ; while mental evolution, in all its stages, may be 
regarded as the psychological aspect of that which, from the 
physiological aspect, is the evolution of nervous systems. Life 
itself is the behaviour of a particular kind of substance which 
is found more or less abundantly under natural conditions. 
No other known substance behaves in this way, and so ignorant 
are we as to the conditions of its natural origin, that it is 
useless to guess at a scientific explanation. And even if we 
knew all the antecedents and conditions of its origin we should 
be no nearer a comprehension of why protoplasm has the 
peculiar properties which we find it to possess. That is a 
question to which science can give no answer. Who kuows 
