1889-90.] 
Professor Calderwood on Evolution. 
77 
ference,” a preference which is the outcome of reflection, forecasting 
possible consequences. 
A reference to the simple diagram already used will illustrate our 
difficulties, when nerve-action and intellectual action are viewed as 
distinct. 
3. Purpose. 4. Action. 
The functions of the sensori-motor system are so well known as 
to help discrimination here. The interaction of sensory and motor 
nerves, under external stimulus alone, gives reflex action. To this 
must be added the phenomena of inhibition, — the restraints which 
a sensory system is capable of placing on its own activity. Next, we 
reckon the action of sense in stimulating appetite, which brooks no 
hindrance, marking one type of “the struggle for existence,” of 
which Darwin has discoursed with fulness. 
What we need to account for is the immense advance occurring 
in life when intelligence interposes to check impulse, and, after 
carrying through a course of reflection, orginates a higher motive, or 
changes the passion into actual motive. We need not here include 
the still greater complexity of action when moral considerations 
are introduced. We restrict the reference to self-interest. 
The intervention of intelligence in the determination of action 
is an- occurrence so different from all that can be traced, even by 
faintest anticipation, in lowest orders of life, that it involves a 
reversal of the law on which the hypothesis of evolution depends. 
Keeping in view here the difference between the lower unintelligent 
animals and the higher intelligent animals, it holds true for all of 
them, that the law of pleasure and pain rules their life. It is 
dominant in the lower as in the higher ; but in the higher, passion 
