14 THE STORY OF THE BIRDS 



climate retreat southwards towards the Tropics, 

 but that they originally extended their range 

 northwards from southern bases where the 

 generic roots of such forms exist to-day, not- 

 withstanding the fact that a glacial catastrophe 

 exterminated all these northern offshoots of 

 which a few scattered relics have been dis- 

 covered. 



The lesson then briefly summarised that all 

 Ornitholites convey is that existing birds — birds 

 as we see and know them to-day, have descended 

 in a direct line from ancestors which differ more 

 and more widely from living types the farther 

 we trace them back into the remote past, until 

 we are confronted by those semi-reptilian forms 

 which appear to be the earliest progenitors of the 

 great and now highly specialised avine group. 



Having thus briefly glanced at the origin of 

 birds let us pass on to a consideration of the 

 most salient points in the structure and char- 

 acteristics of existing forms. Commencing with 

 external features, a bird, as we have already 

 stated, is known by its feathers. What are 

 feathers, those wondrous things with which all 

 birds are clothed, the chief vehicle of their 

 matchless beauty, the bearer of those glorious 

 sheens and tints and hues that single out this 



