88 MIND IN EVOLUTION CHAP. 



impulse to a particular course of action is hereditary, there 

 may be sufficient intelligence, not indeed to devise thewhole 

 plan upon which the animal proceeds, but to carry out 

 those minor modifications which peculiar circumstances 

 demand. This would be an intelligence operating within 

 the sphere of instinct. But there are many modifications 

 which do not seem to require this explanation, and may be 

 ascribed to what we will call pure instinct ; i.e., instinct 

 operating without intelligence. Thus, to begin with, some 

 of the examples of variation adduced above are, as has been 

 already suggested, to be put down to the existence of more 

 than one hereditary reflex mechanism subserving the same 

 end. A normal stimulus A excites the reflex a ; a less 

 frequent stimulus B excites f3 ; the reaction of a to A and 

 to B giving essentially the same result. Thus one sort 

 of danger may cause a creature to run away ; another may 

 prompt it to hide, or perhaps to stay quite still (to " sham 

 dead "). Here we clearly have to deal with hereditary 

 methods of reaction, though they are different methods. 

 But there is a more complex case. Though the whole 

 mechanism employed may be hereditary, and so in a sense 

 reflex, each reflex element may be suitably modified and 

 adjusted by the pressure of the totality of conditions. We 

 have seen already that in some degree this applies to 

 reflexes from an early stage. The replete child does not 

 suck. That is, the reflex excitability depends on certain 

 relevant organic conditions. 1 In a more general way we 

 can now understand how an adjustment, and seemingly a 

 very intelligent adjustment, may be effected to different 

 circumstances through the influence that different stimuli 

 have in counterworking one another. Mr. Romanes tells 

 on the authority of Mr. E. L. Layard a " snake story " 

 which may perhaps be explained in this fashion. 



1 Another excellent illustration of the variation of response according 

 to the general state and requirements of the organism is seen in Prof. 

 Whitman's observations on the leech Clepsine, quoted in Mr. Lloyd 

 Morgan's A nimal Behaviour, pp. 159, 160. This leech has two distinct 

 methods of behaviour in danger. One is to hug the surface on which it 

 is resting, flattening the body and stiffening the flesh. The other is to 

 roll itself up into a ball, free to roll away in any direction. " If by chance 

 the animal has eggs, it will not desert them to escape in this way "- 

 otherwise it may adopt either mode of defence. 



