202 THE WONDER OF LIFE 



which were, of course, embodied in the particular inborn 

 brain-pattern characteristic of the organism in question. 

 He made the further step, which seems nowadays so 

 obvious, of recognizing that in many animals instinctive 

 behaviour predominates, while in others, as in Man, 

 intelligent behaviour predominates. Thus it came to be 

 no longer a question of animal behaviour in contrast to 

 human behaviour, but of two different modes of behaviour, 

 both of which may be illustrated in one creature, to wit, 

 instinctive and intelligent. 



Darwin's contribution comes next. With his charac- 

 teristic common sense, he was quite clear that in animal 

 behaviour we have often to do with individual experiment- 

 ing and inference in other words, with the exercise of 

 intelligence, and often, also, with another kind of capacity 

 instinct * implying some inherited modification of 

 the brain '. 



In the second place, taking cases like the instinct of the 

 young cuckoo to tumble its foster-parents' offspring out 

 of the nest, the instinct of the Ichneumon-fly larvae to 

 devour the soft body of the caterpillar in which they find 

 themselves hatched, the instinct of the cat to play with the 

 mouse, Darwin argued that they were not mysterious 

 implantations, but growths, accumulated and perfected 

 by Natural Selection. As to their origin, he agreed, on 

 the one hand, with the Lamarckian school, that some of 

 them might have arisen through the transmission of intelli- 

 gently acquired habits ; but, on the other hand, he laid 

 most emphasis on Nature's sifting of the inborn variations 

 which are continually cropping up. Variations in structure 

 are of frequent occurrence, and so are variations in the 

 responses that animals make to external stimuli. New 



