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females, and for the control of his harem others are ready at all
one in the specialization of birds and mammals. When the young are cared for the percentage of loss in the struggle for life is greatly reduced, the number of births necessary to the
Fig. 274.—Nest and eggs of the Rufus hummingbird, Trochilus rufus. (Photograph by
maintenance of the species is much less, and the opportunities for specialization in other relations of life are much greater. 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 advantage over polyg. amous animals, and those whose social or mating habits give
Tarla Beads Fig. 275.- Altricial nestlings of the blue jay, Cyanocitta cristata.
rise to a division of labor over those with instincts less highly specialized.
When we study instincts of animals with care and in detail, we find that their regularity 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 machinelike 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 mechanism (the brain and nerves), and in their responsive adaptability.”
Reason or intellect, as distinguished from instinct, is the choice, more or less conscious, among responses to external impressions. Its basis, like that of instinct, 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 becomes reflex. It is essentially like instinct in all its manifestations. The only evident difference is in its origin. Instinct is inherited. Habit is the reaction produced within the individual by its own repeated actions. In the varied relations of life the pure reflex action becomes inadequate. The sensorium is offered a choice of responses. To choose one and to reject the others is the function of intellect or reason. While its excessive development in man obscures its close relation to instinct, both shade off by degrees into reflex