530 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



The moth caterpillar is another noted silk-spinner. Its spinning 

 apparatus is operative from the time it leaves the egg, but only when 

 the caterpillar comes to the end of its feeding life does some internal 

 condition dictate the spinning of a cocoon. Now, without any pre- 

 limmary practice, each caterpillar knows just how to construct a 

 cocoon like that of its predecessors in which to await its dissolution in 

 the pupa. The cocoon is for the protection of the pupa in which the 

 adult moth will be formed. 



Notwithstanding all that has been written on the subject of animal 

 instinct, the physical mechanism of instinctive behavior is still un- 

 known. Simple instincts may be mere reflexes to stimuli, but complex 

 instincts of action and construction, involving a series of coordinated 

 reflexes, are not easily explainable. A single neuromuscular arc may 

 serve for a simple reflex, but it is hardly to be supposed that a complex 

 instinct involves the presence of as many preformed circuits as there 

 are separate acts in the performance. The spinning spider, then, 

 would have to be a complete, automatic web-making machine con- 

 structed to run through the whole series of acts involved in spinning 

 the web. Yet the spider has only one set of muscles, and a different 

 action of the same muscles would have to be stimulated for each act. 

 It would seem, therefore, that there must be some mechanism in the 

 central nervous sj^stem that determines the muscular reaction for each 

 stimulus. This may be what has been called the "innate releasing 

 mechanism," but giving something a name does not explain what it is. 



Some biologists do not like the term "instinct" because it cannot 

 be defined and is often used to mean some mysterious inner sense of 

 the animal. Yet the word is indispensable for convenience in writing 

 about animal behavior ; at least it stands for visible facts. 



As we ascend the ranks of the animal kingdom instinct plays an 

 ever-decreasing role until in man it is almost abolished, except for 

 a few acts such as grasping and sucking during infancy. Among 

 the birds, however, instincts for nest-building are equal to anything 

 in the insect world or the web-spinning of the spiders. Many of the 

 smaller rodents dig burrows that are specific of their kind, and the 

 beaver is noted for the construction of dams and the building of 

 houses. The house cat, though she has no idea of sanitation, care- 

 fully covers her voided feces w^hen out of doors, and w'ill go through 

 the motions even on a wooden floor. Kittens in their play stalk each 

 other and crouch for a final spring, just as did their unknown an- 

 cestors hunting prey in the wild. 



Finally there is the seeming mystery of instinct inheritance. If, 

 however, instinct depends, as it somehow must, on physical structure 

 and organization, it is no more inherited as such than are physio- 

 logical activities, which are functions of inheritable physical organs. 



