WHAT EVOLUTION IS 95 



of the food is given. This kind of 

 response where a second form of 

 stimulus, such as the sound of a bell, 

 replaces the primary stimulus is called 

 by Pawlow a conditioned reflex. To 

 induce this state in untrained mice 

 required, according to him about 

 300 lessons. The descendants of this 

 trained stock, however, acquired this 

 capacity after only 100 lessons, the 

 third generation after 30 lessons, the 

 fourth after 10, and the fifth after 5 

 lessons. Pawlow expressed the hope 

 that in time a generation of mice 

 might be produced in which this con- 

 ditioned reflex would occur immedi- 

 ately and, so far as that generation 

 itself was concerned, without train- 

 ing at all. These statements were 

 published in a preliminary way in 

 1923 and, though in certain respects 

 they are very precise and final, it is 

 hardly possible to comment on them 



