THE YOUNG NATURALIST. 



27 



manner, but in the water every movement is marked by grace and ease. The 

 group of wading birds present another line of development, inasmuch as in 

 seeking their food, it is to their advantage to possess long slender legs and 

 toes, which while enabling them to secure their prey with ease, are yet not so 

 heavy as to impede their flight. A familiar example of structure modified by 

 altered circumstance, is that of the common duck. In the wild state, it 

 possesses extraordinary powers of flight and comparatively weak legs, whilst 

 in domesticated varieties we find these powers reversed, very small wings 

 almost useless for flight but legs largely developed and very strong. Man 

 for his own purposes, has by artificial selection, produced this astonishing 

 change in a short time, are we not then justified in saying that natural selec- 

 tion, operating unceasingly through immense periods of time, has produced 

 the still greater differences, which separate the ostrich from the humming- 

 bird, or which distinguish both from their common ancestor. 



Birds have been not inaptly described as reptiles with feathers. Widely 

 different as the scales of a python and the feathers of a dove appear, yet they 

 are strictly homologous, being in fact an epidermic development. It will be 

 seen that the sloughing of reptiles and the moulting of birds are identical 

 processes. Though in no bird is the moult so complete as in some reptiles, 

 for instance the blind-worm, where the slough is cast in one piece ; yet in 

 some a very near approach to it is made, as in the wild duck, which is for a 

 few days, at the moulting season, incapable of flight, and some of our song 

 birds moult so rapidly as to appear almost naked for a short time. Reptiles are 

 usually spoken of as cold-blooded, and this, though not strictly accurate is 

 nearly so, as in the majority of them the temperature is little higher than 

 that of the surrounding medium. In birds, however, which though structu- 

 rally allied to reptiles, possess a heart similar to that of mammals, and are 

 warm-blooded, exposure to low temperature would be fatal ; and so we find 

 their feathers developed into a protective covering infinitely more effective 

 than the scales of reptiles. This is particularly noticeable in the more aquatic 

 birds, and in those which inhabit high latitudes, in some of which the plum- 

 age protects not only against the weather but against their foes. This latter 

 peculiarity is found however in every clime, and it is owing to the survival 

 and multiplication of those individuals best fitted for their surround- 

 ings, and which is sometimes spoken of as protective mimicry. Striking 

 instances are presented by the ptarmigan, which is white in winter and 

 brownish in summer ; perhaps a more familiar one is the grouse amongst 

 the heather, where only a practised eye can detect its presence. Of further 

 developments of organs for special purposes, we may refer to the beaks of the 

 falcons, and contrast them with the delicate bills of the humming birds - f in 



