MYSTERIES OF LIFE AND EXISTENCE — SNODGRASS 527 



the senior author, "physical and chemical factors are involved, but 

 some organizer appears to be directing these factors." The "or- 

 ganizer," however, does not visibly appear, nor is it evident what the 

 motor force of the transporting cells may be, even if their activities 

 are directed by an organizer, A further mystery is: how did these 

 coordinated activities originate in the evolutionary history of Micro- 

 stomum^ and then become transmitted by heredity ? Some zoologists 

 are reluctant to accept this story of Microstomum and the nematocysts 

 of hydra, but Lewis (1940) and Tinbergen (1951) quote it without 

 comment. 



INSTINCT 



A primarily distinctive quality of animals is mobility, only a few 

 having adopted secondarily a sedentary life by attachment to a sup- 

 port. A moving animal encounters differences in the environment, 

 some of which are advantageous to it and others harmful. Movement, 

 therefore, must be capable of regulation. In some way the animal 

 must be sensitive to environmental conditions, and its sensory impres- 

 sions must be transmitted to the motor system. 



The simplest animal movements are those of the Protozoa. They in- 

 clude avoiding reactions to obstructions, the avoidance of unfavorable 

 chemical conditions in the water, and in some cases reaction to light. 

 The protozoons move either by changes in the shape of the body or 

 by the action of external cilia or flagella. Yet they have no specific 

 sense organs, nervous system, or muscles ; the one-celled body evidently 

 acts in all three capacities. 



The many-celled animals have a great advantage in the potentiality 

 for cell differentiation. This has enabled them to develop specific 

 sensory, conductive, and motor tissues along wdth efficient organs of 

 locomotion. Their capacity for varied movements, therefore, is prac- 

 tically unlimited. In the lower Metazoa, probably including all the 

 invertebrates, the sensory-nervous-muscular circuit works automati- 

 cally in response to a sensory stimulus. The resulting behavior of the 

 animal is called instinct. Instincts, thus, having a purely physical 

 basis, are acquired by structural inheritance, are not learned, and re- 

 quire no act of consciousness on the part of the animal. Instincts are 

 present also in all the higher animals, but in the vertebrates, with the 

 development of the forebrain, the faculty of consciousness appears, 

 leading to intelligence, and instincts become of less and less impor- 

 tance, until in man they are all but suppressed. Animals guided by 

 instinct seldom make mistakes, but they cannot adjust their actions to 

 changed conditions. Animals with intelligence, on the other hand, 

 are prone to make mistakes or to do the wrong thing, but they can 

 correct their errors and adjust their actions to circumstances. 



Tinbergen (1951) in his book on "Instinct" discusses animal be- 

 havior that subserves the individual, such as locomotion, feeding, 



672-174—63 39 



