122 



THE BRITISH NATURALIST. 



[June 



Vertebrata. 



''WHAT IS A BIRD?" 



BY LINN^US GREENING. 



( Conthmed from page 92.) 



Having given a history of the development of a true feathered 

 bird from a scaled reptilian ancestor, it may occur to some to ask 

 what grounds we have for assuming this to be even probably accurate, 

 and is there any analogous process in any other group of animals. 

 To this we must reply that as previously stated, the resemblance 

 between birds and reptiles is so close as to compel the belief that 

 they are divergent members of one family, and that as no better 

 explanation is forthcoming, and the one given is not in opposition to 

 any facts, it may be provisionally accepted, and the assumption that 

 feathers which are now functional for flight, arose primarily as a body 

 covering, is justified by experience. To say that our assumption is 

 justified by experience may sound somewhat startling, but a little 

 consideration of such a familiar object as a butterfly's wing will shew 

 that it IS perfectly just. Without going into minute details it must suffice 

 to point out, that as every entomologist well knows, the structure of a 

 butterfly's wing clearly indicates that it is a modified breathing organ. 

 In other words, what was once the gill of a water insect has become 

 functional solely for locomotion, the breathing has become the flying 

 organ ; and the beautiful colours which adorn our butterfly are due to 

 the scales which .give the name to this order of insects, and which are 

 modifications of the hair-like growth covering the body, which has 

 gradually extended over the wings. This scaly covering of the wings 

 does not enable the insect to fly more rapidly, in fact rather the 

 contrary, and though its original development is due to that process of 

 katabolism which has been previously referred to, yet its preservation 

 and the beautiful arrangements of colour, so characteristic of these 

 insects, must be referred to sexual selection, modified in some cases 

 by what, for want of a better term, is called protective mimicry. The 

 evolutionary process by which our Lepidoptera have been developed 

 from an aquatic worm, is parallel to that by which, as we have 

 endeavoured to shew, the plumage of our existing birds has been 

 developed from the scales of some old world lizards. 



