LIFE-HISTORY. Ixxiii 



singularly disproportionate to their powers of flight. 

 With but few exceptions, their chief progress through 

 the air is by a series of long leaps. A large number 

 of the Delphacidae, and some of the Deltocephalidae, 

 would seem to be losing their wings through a disuse, 

 caused possibly by their unsuitability to the rough 

 winds of the moors, and the marshy plains they usually 

 inhabit. 



Whether this be a vera causa or not, the fact remains, 

 that the presence of such degraded organs is the rule 

 in some species ; whilst side by side with these 

 individuals, but in very reduced numbers, the fully- 

 fledged or macropterous forms are also pretty con- 

 stantly present. 



It is the opinion of some naturalists that a similar 

 degrading cause is operating on certain species of 

 Geometridai, in which, during the late autumn, the 

 pupa3 disclose apterous females. Forests and woods 

 denuded of their leaves are less protected from winds 

 in winter than in summer, and elimination of the 

 wings of these moths at these seasons may be thought 

 to be an advantage to them. 



Mr. Wallace remarks, that insects unable to vary 

 rapidly, would soon die out in favour of those capable 

 of easy adaptation. Again, that "groups of indivi- 

 duals, in their struggle for existence, must adopt new 

 habits to enable them to sustain themselves when the 

 struggle is very severe." 



Changes in structure in such animals will be brought 

 about, because all parts of an animal are assumed and 

 appear to be continually undergoing variation; those 

 variations only being retained which do not clearly 

 conduce to the animal's welfare. 



It will be very interesting to search amongst fossil 

 insects for such transitional forms ; where, as here in 

 the case of the Tettigidae, organs are believed to be 

 undergoing elimination. The occurrence, or otherwise, 

 of such degradation in the ancient geological past, will 

 help us much to make some definite conclusions. 



