■526 Relation of Use and Disuse to Evolution 



This special effect was produced only at the time that the 

 eyes were in process of formation. 



The startling part of this experiment is still to come. 

 The rabbits thus born with defective eyes were allowed to 

 grow up and to breed. For eight generations they continued 

 to reproduce defective eyes, in the manner of a Mendelian 

 recessive, so that it was possible to establish a " pure " strain 

 showing these defects. There would seem here to be a case 

 of parallel induction by the action of a highly specific sub- 

 stance. It is to be noted, in considering this experiment, 

 first, that Stockard and others were able to induce eye de- 

 fects by using non-specific substances, such as alcohol; and 

 second, that Guyer and his associates failed to produce other 

 specific effects although trying various kinds of special sera. 

 Further confirmation of these results and their extension 

 promise at present to give us the most direct answer to the 

 general question. Can a force or agency acting upon an 

 organism and producing alterations in its structures and 

 functions produce in the germ cells such changes as to lead 

 in subsequent generations to the development of similar 

 modifications without the repeated action of the original 

 modifier? 



Pavlov's Experiments 



The supporters of the Lamarckian hypothesis received 

 overwhelming encouragement for their position a few years 

 ago, when the great Russian physiologist Ivan Pavlov visited 

 this country. On the eve of his departure Pavlov referred 

 casually to some experiments in process at his laboratory in 

 Leningrad, where white mice are being conditioned to re- 

 spond to the sound of a bell as they normally respond to 

 the presentation of food. According to Pavlov the normal 

 mice take about three hundred trials to substitute the arti- 

 ficial stimulus for the natural one for arousing the movement 

 toward food. In the second generation the same result was 

 obtained in about one hundred trials, and subsequent genera- 



