APPENDIX. 213 



ascertained routes far away from the place of their 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 Arch&opteryx, which has a long tail furnished with 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 forms widely 

 different from those now living, but they are associated with nearly 

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

 Miocene formations. For further particulars the reader is referred to 

 the Guide to the Geological Gallery. 



The number of species of Birds at present known to exist may be 

 computed at about thirteen thousand. 



