56 HEREDITY AND ENVIRONMENT 



the course of evolution probably because they 

 serve a useful purpose. 



Very much has been written on the nature 

 and origin of instincts, but the best available 

 evidence strongly favors the view that instincts 

 are complex reflexes, which, like the structures 

 of an organism, have been built up, both onto- 

 genetically and phylogenetically, under the 

 stress of the elimination of the unfit, so that 

 they are usually adaptive. 



3. Memory. Another general character- 

 istic of protoplasm is the capacity of storing 

 up or registering the effects of previous 

 stimuli. A single stimulus may produce 

 changes in an organism which persist for a 

 longer or shorter time, and if a second stimulus 

 occurs while the effect of a previous one still 

 persists, the response to the second stimulus 

 may be very different from that to the first. 

 Macf arlane found that if the sensitive hairs on 

 the leaf of Dionaea, the Venus fly-trap (Fig. 

 20, SH), be stroked once no visible response 

 is called forth, but if they be stroked a second 

 time within three minutes the leaf instantly 

 closes. If a longer period than three minutes 



