STRUCTURE OF BIRDS. 149 
ascertained routes far away from the place of birth into milder or 
tropical climates. Recent observations tend to show that tle 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 Archeopteryx, 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 Birds 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 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. 
