FLYING ANIMALS. 27 



for the attachment of the muscles necessary for the 

 down-stroke of the wings. Their mode of flight was 

 probably very similar to that of Bats, which they 

 appear to have resembled in their wing-membranes, 

 although the supports of those membranes, as we shall 

 subsequently see, are arranged on a totally different 

 plan in the two groups. It is, perhaps, superfluous to 

 add that any resemblances existing between Ptero- 

 dactyles and Birds are solely due to their adaptation 

 to a similar mode of life, and that there is not the 

 remotest genetic connection between them. 



We come now to the Birds, in which true flight 

 has attained the fullest development, and the whole 

 organisation is profoundly modified to suit the exigen- 

 cies of a more or less completely aerial mode of life. 

 It is true, indeed, that certain birds, such as the 

 Ostrich, Cassowary, and Penguins, are totally incapable 

 of flight ; but this incapacity is certainly an acquired 

 one in the last-named birds, and there is a considerable 

 probability that it was likewise so in the two former. 



The great peculiarity whereby Birds differ from all 

 other animals is the presence of their external covering 

 of feathers. A feather, as we all know, is one of the 

 most beautiful objects in nature ; and its structure is 

 an admirable instance of adaptation for a particular 

 purpose. The uses of feathers are two-fold. In the 

 first place, the small ones with which the body is 

 clothed form the most perfect covering that can be 

 imagined to ensure the maintenance of the high bodily 

 temperature so essential to the active existence of a 



