232 Habit and Instinct. 



CHAPTEE XI. 



NEST-BUILDING, INCUBATION, AND MIGRATION. 



The activities which were considered in the last chapter 

 are characteristic of a period of high vitality, and one of 

 emotional expressiveness and susceptibility. Whether we 

 accept or reject sexual selection by preferential mating 

 as a factor in the evolution of these specialized activities, 

 it is a matter of observable fact that these activities are 

 coincident in time with the pairing impulse. And it is 

 probable that any selection, which may have been instru- 

 mental in their development, has been in some way associ- 

 ated with the mating instinct. If struggle or competition 

 has occurred, it has centred round the processes essential 

 to the propagation of the race; and the specialized activities 

 of song, dance, or aerial evolutions may be regarded as 

 expressions of the emotional state which accompanies 

 and characterizes the pairing season, and may be held to 

 possess, at least in some cases, a suggestive value. 



There are, however, other activities characteristic of 

 the same period in which the element of suggestion cannot 

 be regarded as of much importance. Nest-building, the 

 instinct of incubation, the maternal offices — these do not 

 depend in any appreciable degree upon a suggestive 

 element. Their direct biological value is, perhaps, more 

 apparent than that of song or dance or strange antics, 

 since they are more obviously of utility. To a considera- 

 tion of some of the activities of this type we will now turn. 



Is nest-building an instinctive activity, or is it a habit 



