110 EVOLUTION OF LIVING ORGANISMS. 



due, as is so often assumed, to some mysterious power 

 of automatically responding favourably to stimulus, but 

 doubtless to the survival of those organisms which re- 

 spond in the right way. To some extent this power of 

 adaptation is possessed by all organisms, and by varia- 

 tion and Selection may lead to the establishment of 

 definite series of responses in several possible directions. 

 Potentially dimorphic and polymorphic species are thus 

 produced both in the animal and in the vegetable king- 

 dom, but while the accommodation of plants is of this 

 simple and direct character that of animals is usually far 

 more complex. 



On the dangers attending specialisation we have al- 

 ready dwelt (p. 108). Advantageous as it may be to 

 acquire a special structure adapted to fulfilling a certain 

 end with rapidity, sureness and precision, yet animals 

 have, so to speak, found it more profitable still to avoid 

 overspecialisation in one or even two or three directions ; 

 to develop a readiness to respond to all sorts of stimuli, 

 and a power of storing up the impression of past responses 

 so as to benefit by "experience" in animals has been 

 built up a system of " behaviour " of such a kind that 

 the character of the final response to the initial stimulus 

 depends on the number and quality of the responses 

 previously called forth. This power has been acquired 

 with the development of elaborate discriminating sense 

 organs, and a complex conducting and co-ordinating 

 nervous system. 



Now this consideration brings us to the threshold of 

 the vast domain of Physiological Psychology, which we 

 cannot fully explore in this little volume. Yet something 

 must be said about Instinct and Intelligence from the 

 point of view of a scientific explanation of Evolution, while 

 taking care not to trespass into the region of Philosophy.* 



* The Mechanistic explanation here adopted should not be confused 

 with Materialism, a discredited system of Philosophy which denies the 

 existence of anything but the material or physical. 



