544 The Outline of Science 



At an early stage there must have been established a number 

 of particular answers (involuntary muscular and nervous move- 

 ments) to stimuli, which became enregistered in the creature, and 

 these ingrained capacities increase in number. We discern a 

 persisting state of the organism which varies the answers ; there is 

 probably a simple expression of conation or endeavour. And in 

 time we come to perceive something of purposive behaviour. 

 "With the establishment of a nervous system there was opened up 

 the possibility of a new kind of organisation that of reflex 

 actions and tropisms, which play an important role in behaviour, 

 an organisation which heredity perpetuates." Reflex actions are 

 automatic movements of nerve-cells and muscle-cells of lower 

 animals, which secure a fit and proper answer to a recurrent 

 stimulus. Tropisms are on a somewhat higher plane; they are 

 forced or obligatory movements of the animal as a whole, that is 

 to say, every creature of the same kind, and in the same 

 physiological state, will behave in the same way. On a still higher 

 level we have instinctive behaviour, 



which reaches its purest expression in ants, bees, and 

 wasps. In birds and mammals it is more likely to occur in 

 co-operation with intelligence. Instinctive behaviour agrees 

 with reflex acts in not requiring to be learned, in being de- 

 pendent on hereditary nervous predispositions, and in being 

 exhibited approximately in the same way by all similar in- 

 dividuals of the species. 1 



We have discussed previously the history of these progressive 

 evolutionary advances, culminating in intelligent behaviour, and' 

 we saw wherein lay their survival value. We need not consider 

 them further here. Reflex actions, tropisms, and instinctive be- 

 haviour have become part of the inborn hereditary constitution of 

 all higher animals. 



The question may be asked, what, besides what we call our 

 mental faculties and our instincts, forms part of our natural in- 



*J. Arthur Thomson, The System of Animate Nature. 



