THE MASTERY OF THE AIR 251 
Hawaii seem to think nothing of setting out 
for Alaska; it means some sense of direction 
that we do not understand, for a young bird 
that has never been more than a few miles 
from home will start gaily in the autumn for 
tropical Africa and will reach its goal in 
safety, and, what is more, will sometimes 
come back again the following spring to the 
precise place of its birth. 
THE FOURTH SOLUTION OF FLIGHT 
The fourth solution of the problem of flight 
was discovered by bats. In its idea it is 
nearest that of the Flying Dragons, but it is 
quite independent and by itself. Bats are, of 
course, true mammals, covered with hair, and 
giving milk to their offspring. They are most 
nearly related to the Insectivores, such as 
shrew and mole, and it is interesting to notice 
that there is in the Far East an aberrant in- 
sectivore called Galeopithecus, placed by 
some authorities in a special order, which has 
a sheet of skin stretched between fore- and 
hind-limbs, and is a very expert parachutist. 
The wing of the bat is formed of a fold of 
skin, which usually begins at the shoulder and 
