142 THE EVOLUTION THEORY 



they hang the egg above it by a silken thread. It is thus in security, 

 and the young larva, too, when it appears, can withdraw to its safely 

 swinging resting-place as soon as danger threatens from the con- 

 vulsive struggles of the unfortunate victim at whose body it is 

 gnawing. 



Every animal has a great many such ' instincts,' which lead it, 

 indeed force it, to act appropriately towards an end, without having 

 any consciousness of that end. For how should the butterfly know 

 what flying is, or that it possessed the power of flight at all, or who 

 could have shown the predatory wasp, when she wakened from the 

 pupa sleep to quite a new kind of life, all that she had to do in order 

 to procure food for herself and to secure shelter and nourishment 

 for the brood which was still enclosed within her ovary? Since 

 species have developed from other species, these regulators of the 

 body, the instincts, cannot have been the same in earlier times ; they 

 must have evolved out of the instincts of ancestors, and the questions 

 we have to ask are : By what factors 1 In what way ? Has the 

 principle of selection been operative here too, or can we refer instincts 

 to the inherited effects of use and disuse ? 



Before I enter upon this question it is necessary to consider for 

 a little the physiological basis of instinct. We can distinguish three 

 kinds of actions: purely reflex, purely instinctive, and purely con- 

 scious actions. In the case of the first, we see most clearly that they 

 depend on an existing mechanism, for they follow of necessity on 

 a particular stimulus, and cannot always be suppressed. Bright light 

 striking our eye makes the pupil narrower by a contraction of the 

 iris, and in the same way our eyelids close if a finger be thrust 

 suddenly towards them. We know, too, the principle of these reflex 

 mechanisms; they depend on nerve connexions. Sensory nerves are 

 so connected in the nerve-centres with motor nerves, that a stimulus 

 aflecting the former at the periphery of the body, as at the eye, is 

 carried to certain nerve-cells of the brain, and from these it excites to 

 activity certain motor centres, so that definite movements are set up. 

 It is rarely only one muscle that is thus excited to activity, there are 

 usually several, and here we have the transition to instinctive action, 

 which consists in a longer or shorter series of actions, that is, of motor 

 combinations. These, too, are originally, at least, set a-going by 

 a sense impression, an external stimulus which affects a sensory nerve 

 exactly in the same way as in the reflex mechanism, and this stimulus 

 is carried to a particular group of sensory nerve-cells in the central 

 nervous organ, and from these transmitted by very fine inter-con- 

 nexions to motor centres. There are extraordinarily complex 



