MECHANICAL NATURE OF INSTINCT 89 



the sense organs, whicli stimulates the peripheral nerve endings, 

 the sensory nerves communicating with the cerebral centres, the 

 cerebral centres themselves, and finally the motor nerves going 

 to the periphery. 



The instinct of self-preservation is of all instincts the most 

 widely and perfectly developed ; and this obvious fact is entirely 

 in harmony with the theory of selection, self-preservation being 

 the first necessity of life. In the majority of cases consciousness 

 cannot even be considered as a possible factor in the origin of 

 these self-preservative instincts ; for it is evident that it is not 

 consciousness, but impulse, which causes the butterfly, as soon 

 as it has left the chrysalis, to fly from an approaching object ; 

 which causes the chicken, as soon as it has been hatched, to pick 

 up seeds ; which causes the young kitten to chase the mouse. 

 For, as we have said, the butterfly can have no knowledge of 

 death, nor can the chicken possibly know by experience that 

 seeds are edible, nor the kitten, which has never seen a mouse, 

 that the mouse is its natural prey. The butterfly flies, the chicken 

 picks up seeds, and the cat chases the mouse, because these 

 external stimuli set in motion certain brain centres which are 

 so constituted that they respond to these stimuli and not to 

 others. 



The instinct of a female butterfly to deposit an egg is awakened 

 only by the sight and scent of certain plants. The appetite of 

 the caterpillar, its instinct for taking food, is likewise excited 

 by certain plants. Place the silkworm on a mulberry-leaf, and 

 it will thrive ; place it on the leaf of a beech-tree, and it will die 

 of starvation rather than eat ; and this although such a leaf would 

 not be in any way poisonous for the silkworm, but simply because 

 the instinct to take food is not awakened by the beech-leaf, 

 whereas it is awakened by the mulberry-leaf. Many species of 

 caterpillars thrive only on one sort of leaf, and a certain beetle 

 species is attracted solely by the deadly nightshade, Atropa 

 belladonna, which is poisonous for other species. The instinct 



