4 A HISTORY OF BIRDS 



There is, in short, no single phase of their life-history which 

 is not throbbing with interest. Ntsts, eggs, and young, ado- 

 lescent and adult, at rest or in action, there is no phase which 

 is not overflowing with matter for reflection, no single incident 

 which is dull ; and this is perhaps more than can be said for any 

 other group of animals. 



Birds, like mammals, display a wonderful plasticity to 

 environment. There is no spot on the earth's surface which 

 has not been made to provide a habitation for them, from the 

 icy and storm-swept regions of the poles, to the equator : while 

 by relatively slight structural changes, they have contrived to 

 avail themselves of food supplies of the most varied description. 



The general homogeneity which birds present, when com- 

 pared with the mammals, or the reptiles, is generally attri- 

 buted to the fact that they are less ancient than either: 

 though they may yet claim a respectable antiquity, since the 

 earliest known bird dates from the Jurassic epoch. It is 

 supposed by some that enormous periods of time are necessary 

 to bring about the extinction of connecting-links, and the con- 

 sequent formation of sharply defined groups, such as are to 

 be met with among the mammalia. But it may well be that 

 this work of extermination among the birds has been largely 

 evaded, and homogeneity preserved, by their ability to escape 

 from unfavourable conditions. With the mammals the avoidance 

 of periods of stress, of whatever kind, has always been restricted. 

 Migration has been limited by unsurmountable barriers ; at one 

 time mountain ranges, at another of vast stretches of water. 



To this evasion of Fate most modern taxonomers owe 

 their difficulties in their attempts to classify birds, that is to say, 

 those who desire merely to establish so many well-defined 

 groups which can be readily summarised in the form of a 

 " Key ". Those, on the other hand, who have spent laborious 

 days in the endeavour to trace the descent of the more or less 

 well-marked groups of living and extinct birds, though hardly 

 more successful than their less ambitious neighbours, must look 

 to yet other and more subtle factors. First, and foremost, 

 must be placed the fundamental modification due to the ex- 

 igencies of flight, which all birds, without exception, share in 

 common. Thereby the range of possible variation has been 

 kept within very narrow limits, and secondly, there must be 



