INTERNAL ANATOMY. 41 



for flying in the bird, and fins for swimming in the fish. 

 It is not at all Ukely that the wings of insects were at 

 first the perfect organs we are acquainted with, and 

 they probably had a similar history. In the course of 

 their descent from wingless animals the laws of evolu- 

 tion show us that insects must have passed through a 

 period in which the wings gradually arose, first as pads 

 similar to the sac-like pads of the young of existing 

 insects, and then through many generations, and by 

 the introduction of progressive modifications, these 

 awkward- looking, stiff appendages became efiicient 

 aerial supports similar to those we admire for their 

 lightness, complicated structure, and graceful outlines 

 in the living animals.^ We can understand upon this 

 theory that leaping insects, in striving to use every 

 means in their possession for levitation, may, by effort 

 through many generations to use their pads, have de- 

 veloped them into wings and also enlarged and modi- 

 fied parts of the tracheal system and brought it into 

 use in producing a more perfect wing than could other- 

 wise have been practicable. It is possible to account 

 for the way in which insects expand the crumpled 

 and thickened wings with which they enter upon the 

 imago state, by this theory, and to explain their habits 

 at this period, especially the deliberate and prolonged 

 efforts to pump air into the wings and expand them into 

 form before they can become dry and hard. We can 

 also understand how the reversal of this process could 

 have made the Lubber locust the heavy, unwieldly 

 thing it now is, through its habit of feeding vora- 



- See p. 79. 



