66 INSTINCTIVE BEHAVIOUR 
and so defines instinct as to include acquired habits. “ Move- 
ments,” he says,* “which originally followed upon simple or 
compound voluntary acts, but which have become wholly or 
partly mechanized in the course of individual life, or of generic 
evolution, we term zstinctive actions.” In accordance with 
this definition, instincts fall into two groups. Those “ which, 
so far as we can tell, have been developed during the life of the 
individual, and in the absence of definite individual influences 
might have remained wholly undeveloped, may be called acquired 
instincts.” They have become instinctive through repetition. 
“To be distinguished from these acquired human instincts are 
others which are connate.” Now, there can be no question 
that behaviour which has become habitual through frequent 
repetition is frequently, in popular speech, described as in- 
stinctive. We hear it said that the experienced cyclist guides 
his machine instinctively. And the word is similarly used in 
many like cases. But we shall find it conducive to precision 
and clearness of thought to emphasize the distinction between 
what is acquired in the course of life and what is congenital in 
the race. And to this end we shall regard behaviour which 
has ‘become mechanized in the course of individual life” as 
due to acquired habit, reserving the term instinctive for such 
behaviour as is independent of individual experience. We 
shall, in short, so far accept Spence’s definition. 
In this definition, as in those of the majority of naturalists, 
it seems to be further implied that instinctive behaviour is of 
a relatively definite kind, though it is no doubt subject to such 
variation as is found in animal structure and organization. 
Mr. Rutgers Marshall, however, in a recent work,{ protests 
against any such implication, and urges that “this variableness 
is so wide that definiteness of reaction cannot for a moment be 
used as a differentia in relation to instinct without narrowing 
our conception of the bounds of instinct in a manner to be 
deplored.” “The actions,” he says, “connected with the 
preparation for self-defence, those connected with protection of 
* « Lectures on Human and Animal Psychology,” pp. 388, 397, 399. 
+ “Instinct and Reason,” pp. 90, 92. 
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