THE CLASSIFICATION OF BIRDS 17 



Unfortunately, in too many cases it is impossible to decide 

 which of two characters is to be ascribed to analogy and which to 

 affinity, and thus the classification of Birds becomes to a consider- 

 able extent a matter of guess-work. All that can be done is to 

 go on guessing and to finally accept the combination which pre- 

 sents the fewest in number and the least important in character 

 of instances of the independent acquisition of any modification. 



In most recent attempts to make a natural classification of 

 Birds, the families have been arranged so as to place the most 

 highly developed Birds (presumably the Passeres) at one end of 

 the series, and the least highly developed (possibly the Struthiones) 

 at the other. This arrangement appears to me to be artificial, 

 and has probably done much to make a natural classification 

 impossible. Of the half-dozen (more or less) subclasses into 

 which the surviving species of Birds may be grouped, most of 

 them, if not all, may contain families in various stages of develop- 

 ment, which we in our ignorance may be pleased to term highly 

 developed or otherwise. In many cases the apparently archaic 

 forms may be very highly specialised for some peculiar niche in 

 the organic world into which they have been squeezed during the 

 struggle for existence, and which they exactly fit. 



The fact that all Birds have feathers is some argument for the 

 assumption that the ancestors of all Birds once possessed the 

 power of flight, otherwise their feathers would be of little use to 

 them. It is difficult to imagine how the power of flight could 

 have been acquired except by some animal which lived in a tree 

 and began by jumping from bough to bough ; hence it seems a 

 reasonable assumption that the adaptation of the wing of the 

 Penguin to serve as a paddle under water is not an archaic 

 character which has been retained. It may be the partial 

 resumption of an archaic character which has been independently 

 reproduced, but is probably more correctly described as a highly 

 specialised modification of later date adapted to special ends. 

 The same may be said of the apparent degradation by disuse of 

 the wing of the Ostrich. 



Birds having been originally perchers in trees, we may accept 

 the Passerine foot as well as the Passerine wing as the normal 

 avian type. In the struggle for existence it is not surprising 

 that Birds should have adapted themselves, or should have been 

 adapted by surrounding circumstances, to other modes of life ; 

 and we consequently find that Birds may be classified into various 



B 



