Heredity of Instincts. 2 1 



ground, a circumstance which gave rise to the erroneous idea that 

 nature's treatment of the animal had been that of a stepmother. 

 But this is not the case : the sloth is as perfect in its kind 

 as all other animals ; its limbs are so arranged as to enable it to 

 climb, and to live in trees. The spider's legs are so arranged and 

 organized that it moves with difficulty over a plane surface : these 

 organs are intended for use on a line or a thread, and the spider 

 carries about the materials from which to spin such thread. 1 In 

 general we might say : As is the organism, so are the instincts ; 

 and vice versa. Given the instincts of an animal, a good naturalist 

 can infer its organization j or, given its organization, he can infer 

 its instincts. 



This intimate correlation between the physiological and the mental 

 constitution leads naturally to the conclusion that the instincts 

 of an animal result from its organization. Each organ, even each 

 tissue, has its special function to discharge, and this tendency to 

 the discharge of functions constitutes the need or instinct; the same 

 organ or the same tissue communicates to the being in which it 

 exists this same need ; each additional organ or tissue adds a 

 new need or instinct. Hence the instinct of an animal is the sum 

 of the instincts of its various organs ; it is their necessary their 

 inevitable consequence, and it comes into play under influences 

 to which the animal is unconsciously subject. 



This explanation is simple enough, but may not be perfectly 

 sound. It is certain that instinct depends on organization, but it 

 is very questionable whether it results exclusively from it. This is 

 a region where the phenomena are so complex that physiology 

 is insufficient to explain them all, for here evidently occurs the 

 mysterious transition from the purely organic to the mental life, by 

 means of reflex action, which is principally physiological, and of 

 instinct, which is principally psychological. This transition is 

 insensible and incomprehensible, and serves well to show that any 

 line of demarcation drawn between psychology and physiology is 

 arbitrary, and that mental life is slowly and gradually disengaged 

 from physical life, so that it is impossible to tell where or how 

 it has its rise. Neither can mechanism which seems to be the 



1 Miiller, Physiologic, ii. 108. 



