STRUCTURE OF BIRDS. 149 



ascertained routes far away from the place of birth into milder or 

 tropical climates. Recent observations tend to show that the farther 

 north a species breeds in the Northern Hemisphere, the higher is the 

 southern latitude in which it passes the northern winter. Immense 

 distances are thus traversed by some migrants twice in every year, in 

 their northern and southern movements. 



Geological History . Our knowledge of the geological history of Birds 

 is very scanty. The oldest known bird from Jurassic formations is the 

 remarkable Archceopleryx, which has a long tail furnished \vith a row of 

 feathers on each side. A number of swimming and wading Birds lived 

 in the Cretaceous period, and in some of these the jaws were furnished 

 with teeth. Among the Tertiary Birds there are many Birds widely 

 different from th*)se now living, but they are associated with nearly 

 all the principal types now in existence. The majority occur in 

 Miocene formations. lA>r further particulars the reader is referred to 

 the ninth edition of the " Guide to the Fossil Mammals and Birds/' 

 p. 86 (1909). 



The number of species of Birds at present known is probably not 

 less than thirteen thousand. 



