26 BBITISH CICADA. 



such and other considerations, Mr. Darwin and Mr. 

 Wahace raise arguments, that species are only strongly 

 marked and permanent adaptations, and that com- 

 munity of descent is necessarily implied in the word 

 variety. (See 'Origin of Species,' p. 120.) 



But although Dr. Herman Midler once counted 

 thirty-five distinct variations in two hundred flowers 

 of Myosurus minimus, he did not doubt that his speci- 

 mens were those of the common mouse-tail ; and so it 

 may be said that every zoologist knows, by a sort of 

 instinct, what he means by a species. Deductions as 

 to the changeable form of the segments of the Delpha- 

 cidsB with their genital valves, perhaps may, in a limited 

 sense, be referred to what is now know^n of the 

 opercular valves of some rock-barnacles {Pijroma). 



Both Mr. Darwin and Mr. Wallace have sought an 

 explanation of the descent of groups of species, in the 

 circumstance that such groups have been modified by 

 a process of natural selection from ancestors less 

 specialized. 



It will be difficult to state any satisfactory hypothesis 

 with reference to the arrested development of many 

 Delphacidse, and the cause of the persistent rudimentary 

 characters of their elytra. Together with the rare 

 occurrence of the fully-winged forms, almost invariably 

 we find side by side with them forms nearly apterous. 

 Perhaps some light may be thrown on this question by 

 observations brought forward by Mr. Darwin on the 

 subject of the "Disuse of parts." (See 'Origin of 

 Species,' p. 109.) He suggests that the occurrence of 

 so many wingless butterflies in Madeira may be due to 

 the circumstance that security against the high winds, 

 which would force insects out to sea, would, by an 

 advantageous selective degradation of wing, lead them to 

 less aerial habits. Thus a degradation from disuse would 

 supervene, and the winged forms slowly would die out. 



The question now arises. Are the voluminous wings, 

 possessed by certain individuals of the Delphacidae, 

 advantageous or injurious to their economy ? 



