CHAP. XIV.] ITS NATURE AND ORIGIN. 235 



occur in nature. . . . Under changed conditions of life it 

 is at least possible that slight modifications of instinct 

 might be profitable to a species ; and if it can be shown 

 that instincts do vary ever so little, then I can see no diffi- 

 culty in natural selection preserving and continually accu- 

 mulating variations of instinct to any extent that was profit- 

 able. It is thus, as I believe, that all the most complex 

 and wonderful instincts have originated. As modifica- 

 tions of corporeal structure arise from, and are increased 

 by, use or habit, and are diminished or lost by disuse, so 

 I do not doubt it has been with instincts. But I believe 

 that the effects of habit are in many cases of subordinate 

 importance to the effects of natural selection, of what may 

 be called spontaneous variations of instincts : that is of 

 variations produced by the same unknown causes which 



produce slight deviations of bodily structure For 



peculiar habits confined to the workers or sterile females, 

 however long they might be followed, could not possibly 

 affect the males and fertile females which alone leave 

 descendants." 



As typical instances of the more complex Instinctive 

 Acts may be cited the web-weaving and nest-building 

 habits of Spiders ; the gathering and storing of honey, 

 together with all the social acts of Bees ; the slave-making 

 and other habits of Ants ; the migrations of Fishes at 

 spawning-time ; the selection of site and mode of ovi- 

 position among Amphibia; the nest-building acts and 

 migrations of Birds ; the house-building and food-storing 

 acts of Beavers. There can be little doubt, that if our 

 means of knowledge were greater than it is, we should 

 be able to explain these and all other Instincts by refer- 

 ence to the doctrines of 'inherited acquisition' and 

 'natural selection,' either singly or in combination. 



