460 R. T. YOUNG 



of ascertaining whether or not the harmony of the frogs' color 

 with that of their background was really protective to them. 

 I at first designed to keep the crows confined in a closed compart- 

 ment at the- top of the cage in order to prevent their observing 

 the placing of the frogs and the consequent fastening of their 

 attention on one of them. I soon found that this proceeding 

 was unnecessary, however, as the crows delayed for several 

 minutes, in some cases more than half an hour, before dropping 

 to seize their prey, and gave no evidence whatever of having 

 their attention fixed on either of the frogs at the time these 

 were placed upon the backgrounds. In some instances, the frogs 

 would move after being placed in position, thus dra\^dng the 

 attention of the crows. Where this occurred, I have noted it 

 in the account of the experiment. A further difficulty was 

 experienced in the high wind to which the cage was at times 

 exposed during the experiments, which blew the loose sand over 

 the frogs, changing to a considerable extent their color, as well 

 as that of the mud background. A third difficulty was the 

 •change of color which the frogs underwent after being stunned, 

 and also after being removed from the dark box, in which they 

 were kept, into the light. Thus their color, in addition to the 

 individual differences, was not constant in all of the experiments. 

 The sum total of these influences, while introducing uncertainty 

 into the results, tended to diminish rather than to increase 

 the protective influence of the frogs' color. Hence my results, 

 tending to prove such influence under these disadvantageous 

 conditions, should be at least accepted at par value, rather than 

 discounted in consequence of such conditions. 



Experiments 1 and 2. One specimen of cantabrigensis was placed 

 on mud and one on sand in each experiment. In each the frogs were 

 taken from the sand. In the second experiment the frog on the sand 

 moved just as I left the cage, so that the crow's attention was possibly 

 drawn to this frog rather than to the other. This is not probable 

 however as the crow delayed seizing it for several minutes after I left. 

 After seizing this frog, that on the mud background moved, and the 

 crow dropping the first frog, seized the other one. 



Experiment 3. One specimen of pipiens was placed on grass and 

 one on sand. A few minutes later a crow dropped on to the grass, 

 but took the frog from the sand. 



