8 INTRODUCTION 
in regarding them as unconscious automata and should 
proceed to cut them up in a ruthless fashion on that as- 
sumption. It may be, as Norman has attempted to show, 
that pain sensations in the lower invertebrates are not 
acute at most, and may be absent entirely, but not much 
would be lost by giving the animals the benefit of the 
doubt so far as is consistent with attaining the objects of 
experimental work. 
The scientific instinct for continuity tempts us to ascribe 
some form of sentiency to all living forms. Certainly it is 
not possible to disprove the existence of consciousness of 
some sort in any organism. In the lowest animals it may 
be nothing more than a very dull form of sensibility. If we 
would avoid: the assumption of an absolute beginning of 
consciousness we may hold that something akin to con- 
sciousness, but very much more primitive than any of the 
forms of it with which we are acquainted, exists in connec- 
tion with all inorganic processes. Such a position affords 
a consistent viewpoint and has been accepted by many as 
enabling us to conceive the whole process of psychic evolu- 
tion as one without any breaks or discontinuities anywhere 
in the series. 
Our task in the present volume is to sketch briefly the 
evolution of behavior from its simplest manifestations 
in the reflex actions of the lowest organisms to its more 
elaborate expressions in the higher mammals. No attempt 
has been made to give a survey of behavior in all the groups 
of the animal kingdom. Beginning with simple reflex action 
we have described those forms of behavior which are com- 
monly called tropisms and which are by many regarded as a 
comparatively direct outcome of reflex irritability. After 
an account of behavior of the lowest members of the ani- 
mal kingdom we have given a general account of instinct, 
