20 ANIMAL. LIFE PAST AND PRESENT. 



The special modification of the first pair of legs to 

 subserve the purpose of flight in those Vertebrates 

 which possess this power in its true form, may be 

 taken as an indication that such Vertebrates have 

 originally descended from others in^which that power 

 was not developed. Although we have no such guide 

 in the case of Insects, yet the circumstance that in all 

 those kinds which undergo a complete metamorphosis 

 no traces of wings are observable in their larvae, points 

 with equal clearness to the conclusion that these 

 creatures have been likewise derived from crawling 

 ancestors, and that their power of flight is an acquired 

 one. Those Insects which are unable to fly must not, 

 however, be regarded as ancestral forms, since there is 

 clear evidence that their wings have been lost or have 

 become rudimentary. It has been already mentioned 

 that while all flying Vertebrates have only a single 

 pair of wings, many Insects are provided with two 

 pairs of these organs ; and from the tendency among 

 Insects for one or other of these pairs of wings either 

 to disappear or to be modified for other purposes, it 

 would appear that a single pair is decidedly the best 

 suited for flight. 



We shall now proceed to trace some of the chief 

 modifications in the organs of flight in the different 

 groups of animals, commencing with Insects, in which, 

 as already observed, we never meet with spurious 

 flight. 



In all the Beetles, or Coleoptera, which form the 

 first order of Insects, the front pair of wings are modi- 



