DIVISIONS OF BIRDS. 291 
against becoming wet whilst in the water by the great develop- 
ment of the oil-gland at the tail, by means of which the dense 
plumage is kept constantly oiled. As a rule, the Natatorial 
birds are polygamous, each male having several females ; and 
the young are hatched in a condition not requiring assistance 
from their parents, being able to swim about and procure food 
for themselves the instant they are liberated from the egg. 
TT hy 
& 
i 
——- 
a e 
Fig. 155.—Natatores. Penguin (Spheniscus demersus). 
Amongst the more important families of the Va/atores may 
be enumerated the Penguins (Sphenzscide), the Auks (Alcide), 
the Gulls and Tegns (Lard), the Petrels (Procellaride), the 
Pelicans (Pelicanus), the Cormorants (Phalacrocorax), the Gan- 
nets (Sz/a), the Ducks (Axatine), the Geese (Anserine), and 
the Swans (Cygnide). 
The Penguins and Auks, with their allies the Divers, Guille- 
