2 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
Looked at from the physiological standpoint, and regard- 
ing merely the different ways in which birds have lost the 
faculty of flight, and the conditions appertaining thereto, 
all birds so circumstanced may be divided into three 
groups. 
(1) Birds which have lost the use of their wings (or, 
according tc some authorities, which have never acquired 
them) because the great development of their posterior 
extremities, whether for the obtaining of food or the 
avoidance of enemies, has rendered the use of their 
anterior extremities unnecessary to them. This group 
may be sub-divided into two, according as the posterior 
extremities have been developed for progression, (a) in 
water, (b) on land. The first sub-division of this group 
(that developed for aquatic purposes) contains one bird 
only—the gigantic extinct Hesperornis ; whilst the second 
sub-division comprises the Ostrich and its congeners, 
indeed, all birds belonging to the Ratite sub-class. q 
(2) My second group comprises birds in which the 
anterior limbs are perfectly useless for flight, but which 
have, nevertheless, had these organs specially developed 
for progression in another medium, to wit, water, for all 
birds of this group are aquatic. Under this head fall all 
the Penguins, and probably the extinct Great Auk. 
(3) Yet another group comprises birds from very 
different families, in which the anterior extremities or 
wings are perfectly functionless, but which have, never- 
theless, not acquired any compensatory advantages in 
other directions. This includes degenerate birds, such as 
Didus, Stringops, &c. 
I need scarcely remind you of the customary classifica- 
tion by which the great class Avus is divided into two 
main sub-classes—the CARINAT# and the RatiT#&, accord- 
ing as the sternum possesses a keel or the reverse; the 
