66 



The May-flies and Stone-flies 



fall to the water's surface and there are swept along by wind and wave, 



until finally cast up in thick winrows, miles long, 

 on the lake beach. Millions of dead May-flies 

 are thus piled up on the shores of the Great 

 Lakes. 



We call the May-flies the Ephemerida, after 

 the Ephemerides of Grecian mythology, and the 

 name truly expresses their brief existence above 

 water. But they have lived for a year at least 

 before this, or for two or even three years, as 

 wingless, aquatic creatures, clinging concealed 

 to the under side of stones in the lake or stream 

 bottom, or actively crawling about after their food, 

 which consists of minute aquatic plants and animals 

 or bits of dead organic matter. In this stage their 

 whole environment, habits, and general appearance are 

 radically different from those of the brief adult life. We 

 can only guess, if our curiosity compels us to attempt some 

 explanation, at the manner and the cause of such a 

 strange life-history. What advantage is there in such a 

 specialized condition that Nature could not have arrived 

 at by less indirect means ? What is indeed the utility of 

 the whole modification ? The quick answer " utility," 

 which is to account for all such strange structural and 

 physiological conditions on the basis of useful adapta- 

 tions brought about by the slow but persistent action 

 of natural selection, leaves us, confessedly, answered 

 simply on a basis of belief. In hundreds of cases that 

 may come under our observation, in how few are we 

 really able to perceive a reason-satisfying course of adap- 

 tive development based on the selection of useful small 

 fluctuating variations? 



The eggs of the May-fly fall from the body of the 

 mother to the water's surface in two packets, which, 

 ft however, break up while sinking, so that the released 



FIG. 101. May-flies about an electric lamp. 



