440 Mental Factors in Evolution 
no doubt they are closely related and have some common factors. 
It is probable that play, as preparatory behaviour, differs in bio- 
logical detail (as it almost certainly does in emotional attributes) 
from the earnest of after-life and that it has been evolved through 
differentiation and integration of the primary tissue of experience, 
as a preparation through which certain essential modes of skill may 
be acquired—those animals in which the preparatory play-pro- 
pensity was not inherited in due force and requisite amount being 
subsequently eliminated in the struggle for existence. In any case 
there is little question that Prof. Groos is right in basing the play- 
propensity on instinctive foundations. None the less, as he contends, 
the essential biological value of play is that it is a means of training 
the educable nerve-tissue, of developing that part of the brain which 
is modified by experience and which thus acquires new characters, of 
elaborating the secondary tissue of experience on the predetermined 
lines of instinctive differentiation and thus furthering the psycho- 
logical activities which are included under the comprehensive term 
“intelligent.” 
In The Descent of Man Darwin dealt at some length with intelli- 
gence and the higher mental faculties? His object, he says, is to 
show that there is no fundamental difference between man and the 
higher mammals in their mental faculties; that these faculties are 
variable and the variations tend to be inherited; and that under 
natural selection beneficial variations of all kinds will have been 
preserved and injurious ones eliminated. 
Darwin was too good an observer and too honest a man to 
minimise the “enormous difference” between the level of mental 
attainment of civilised man and that reached by any animal. His 
contention was that the difference, great as it is, is one of degree 
and not of kind. He realised that, in the development of the 
mental faculties of man, new factors in evolution have supervened— 
factors which play but a subordinate and subsidiary -part in animal 
intelligence. Intercommunication by means of language, approbation 
and blame, and all that arises out of reflective thought, are but fore- 
shadowed in the mental life of animals. Still he contends that these 
may be explained on the doctrine of evolution. He urges* “that man 
is variable in body and mind; and that the variations are induced, 
either directly or indirectly, by the same general causes, and obey 
the same general laws, as with the lower animals.” He correlates 
mental development with the evolution of the brain‘. “As the 
various mental faculties gradually developed themselves, the brain 
1 The Play of Animals, p. 24. 
? Descent of Man (1st edit.), Chaps. 1, ut, v; (2nd edit.), Chaps. mu, rv, v. 
8 Ibid. Vol. 1. pp. 70, 71; (Popular edit.), pp. 70, 71. 4 Ibid, p. 81. 
