CHAPTER VII 
THE EVOLUTION OF ANIMAL BEHAVIOUR 
I.—TueE PuysriotogicaL ASPECT 
AT the outset of our inquiry, we used the word “ behaviour ” 
in a wide and comprehensive sense. Thus broadly used, I 
said, the term in all cases indicates and draws attention to the 
reaction of that which we speak of as behaving in response 
to certain surrounding forces or circumstances which evoke 
the behaviour. The behaviour of living cells is dependent on 
changes in their environment; that of deciduons trees, as 
they put forth their leaves in the spring or shed them 
in the autumn, is related to the change of the season; in- 
stinctive, intelligent, and emotional behaviour are called forth 
in response to those circumstances which exercise a constrain- 
ing influence at the moment of action. Used in this com- 
prehensive sense, the term “behaviour” neither implies nor 
excludes the presence of consciousness. We know from our 
own experience, however, that consciousness does in some 
cases accompany behaviour, and we infer that in many other 
cases it may be present. But we need a criterion of its 
presence to guide our inferences, and this criterion we found 
in the ability of living beings to profit by experience. In 
Dr. Stout’s phraseology, if a thing seems to acquire meaning 
for such a being, and the behaviour is guided in accordance 
with such acquired meaning, we infer the presence of con- 
sciousness as supplying conditions effective in determining 
its course. Still this does not exclude, nay, rather it pre- 
supposes, the presence of sentience at a lower stage of evolution, 
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