EVOLUTION OF INTELLIGENT BEHAVIOUR 167 
food, and, on the other hand, the intentional isolation of these 
qualities for the purposes of thought and rational explanation. 
Abstraction they regard as a deliberate process applied with 
rational intent to the material afforded by experience and 
reflection. Generalization, too, they regard as deliberate, and 
carried out with like intent. The result is not merely a 
composite or generic product, but something more subtle and 
less dependent on sense. ‘‘ All trees hitherto seen by me,” 
said Noiré, “leave in my imagination a mixed image, a kind 
of ideal presentation of a tree. Quite different is my concept, 
which is never an image.” The concept “tree ” is a deliberate 
synthesis of abstract qualities intentionally isolated, and 
recombined in accordance with the general relationships which 
subsist between them. 
If we accept this distinction, if we regard abstraction and 
generalization as intentional mental processes carried out with 
the rational intent of discovering the relationships of pheno- 
imena with the object of explaining them and recombining 
their essential features in an ideal scheme of thought, we shall 
probably admit, with John Locke, that these are excellencies 
which the faculties of brutes do by no means attain to. But 
we shall none the less see that the predominance of certain 
salient features in experience by reiterated emphasis in 
association with natural needs, and the development of generic 
in place of merely particular re-presentations will afford the 
appropriate material for abstraction on the one hand, and 
generalization on the other. Intelligence supplies the em- 
bryonic mental structures from which, under the quickening 
influence of a rational purpose, abstract and general ideas may 
be evolved. 
The essential features of the evolution of intelligence seem, 
then, to be, first, the development of controlling nerve-centres, 
by which the responsive action of reflex automatic or instinc- 
tive centres may be checked, augmented, or modified ; secondly, 
the increased differentiation and integration of these control 
centres with extension of the range and complexity of ex- 
perience in close touch with practical needs; thirdly, the 
