INSTINCT AND REASON 251 
In these regards, the nest-building and home-making 
animals have the advantage over those that have not these 
instincts. The animals that mate for life have the advan- 
tage over polygamous animals, and those whose social or 
mating habits give rise to a division of labor over those 
with instincts less highly specialized. 
The interesting instincts and habits connected with nest 
or home building and the care of the young are discussed 
in the next chapter. 
139. Variability of instincts —When we study instincts 
of animals with care and in detail, we find that their regu- 
larity is much less than has been supposed. There is as 
much variation in regard to instinct among individuals as 
there is with regard to other characters of the species. 
Some power of choice is found in almost every operation of 
instinct. Even the most machine-like instinct shows some 
degree of adaptability to new conditions. On the other 
hand, in no animal does reason show entire freedom from 
automatism or reflex action. “The fundamental identity 
of instinct with intelligence,” says an able investigator, “is 
shown in their dependence upon the same structural mech- 
anism (the brain and nerves) and in their responsive adap- 
tability.” 
140. Reason.— Reason or intellect, as distinguished from 
instinct, is the choice, more or less conscious, among re- 
sponses to external impressions. Its basis, like that of in- 
stinct, is in reflex action. Its operations, often repeated, 
become similarly reflex by repetition, and are known as 
habit. A habit is a voluntary action repeated until it be- 
comes reflex. It is essentially like instinct in all its mani- 
festations. The only evident difference is in its origin. 
Instinct is inherited. Habit is the reaction produced with- 
in the individual by its own repeated actions. In the 
varied relations of life the pure reflex action becomes inade- 
quate. The sensorium is offered a choice of responses. To 
choose one and to reject the others is the function of intel- 
