40 



Heredity and Environment 



cells, and of the simplest organisms, to stimuli are in the nature 

 of reflexes or tropisms, that is, relatively simple, machine-like 

 responses. "Reflex motions" originally referred to those re- 

 sponses of higher animals in which the peripheral stimuli were 

 reflected, as it were, from the spinal cord to the appropriate muscles 

 without the participation of the brain. But at present the word 

 "reflex" has come to have a much broader application and is used 

 for all simple, automatic responses, even though there are no 

 nerves and even when the response is not movement but secretion, 

 metabolism or any other activity. "Tropisms," on the other hand, 

 is a more specific term and refers to the movements of organisms 



a 





19 



■ - - ' - 



. « - , 



19* 



19 







26 ' 



38' 



Fig. 17. Reactions of Paramecium to Heat and Cold. At a the in- 

 fusoria are uniformly distributed in a trough, both ends of which have a 

 temperature of 19 ; at b the infusoria are shown collected at the cooler 

 end of the trough; at c they have collected at the warmer end of the 

 trough. (From Jennings, after Mendelssohn.) 



