and the Elements in which they live. 391 
in the less extensive development of their wings, in the more 
scale-like form of their feathers, and the greater number of eggs 
they lay, and the less care they take of their young, which are 
hatched in a state of development in which they are already 
prepared to provide for their own food. The same is the case 
with the Gallinaceous and the Wading Birds, which, though more 
advanced in many respects, are still inferior to the climbing and 
Passerine Birds in this respect, having a heavier flight, if they fly 
at all, and living a more terrestrial, and even aquatic life; the 
Wading Birds coming nearer in this respect, to those with palmate 
fingers, and the Gallinaceous Birds, as well as the Ostriches hav- 
ing a more terrestrial mode of life, whilst the Passerine Birds rank 
higher in all these respects, feed their young, and take care of 
them for a longer time, and live almost exclusively an aérial life, 
few of them having aquatic habits, and those being in their 
respective families by their form as well as by their mode of life, 
decidedly inferior to their loftier relations. 
_ The classification of Birds as a whole is still so imperfect, 
though their minor groups are well understood, that many impér- 
tant relations in these respects must necessarily be more or less 
concealed as long as their primary divisions are not better known ; 
So that we may expect many interesting hints from further inves- 
tigations in this view. . 
_ The class of Mammalia is not only the most diversified in 
the forms of its members, but also in the diversity of their mode 
of life; nevertheless this diversity is connected by the most in- 
timate relations of structure. ‘The Whales are as much Mam- 
malian by their internal organization as the most exclusively ter- 
restrial quadrupeds. T'rue Cetaceans constitute a natural family, 
all the members of which are exclusively marine, and no one 
of them even fluviatile—for the Sirenidee must be considered as 
entirely distinct from true Cetaceans; and these Cetaceans, at the 
same time that they are so exclusively marine, are also the low- 
est type of Mammalia, not only from the imperfection of their 
extremities, of which there is only one anterior pair, and from the 
Want of hind-legs, but also from the extraordinary development 
and bulk of their muscular tail, and the development of a caudal 
n, and sometimes even a fin-like fold of the skin upon the back. 
If it can be shown that the Sirenide are an aquatic type of a 
larger group embracing Pachyderms, the direct relation of their 
Structure and mode of life will be at once obvious, since Sirenidee 
are either marine or fluviatile, while trae Pachyderms are terres- 
trial ; and should we not be justified in considering the subaquatic 
ippopotamus as inferior to its more terrestrial relatives of the 
genera Rhinoceros, Elephant, and Horse? Are we not to consider 
the Ornithorhynchus, with its palmate hind-legs and spur, as infe- 
tior to Echidna? Are not the palmate Rodentia inferior to the ter- 
