and its Significance in Systematic Zoology. 87 
general; and commencing with cases of the greatest simplicity, 
let us pass on to those which are more complex. 
Now, among all the Vertebrate Classes there are certain 
general homologies which structurally unite every animal con- 
tained within them, however it may differ in external form. In 
all, the diverging appendages are present in some form or other, 
except, indeed, in certain Ophidians, in which they are entirely 
absent. In Birds, the modification of the fore extremity is 
obvious, and in Fishes only somewhat less so; but, although 
the relative position of the pectoral and ventral fins is some- 
times reversed (as in the Perch, for example), still the pectorals 
are always homelogous with the fore, and the ventral with the 
hind limbs of other Vertebrata. There is therefore a great com- 
munity of plan in Vertebrates, with respect to those parts which 
constitute the elements of external form. 
Let us now glance at the media in which they move. Mam- 
-malia are, as a class, destined to tread the surface of the earth, 
birds to fly in the air, and fishes to swim in the sea: but 
neither is the air nor the sea devoid of Mammalian inhabitants ; 
and both land and water, as well as air, afford a home for birds. 
Reptiles also occupy all three stations ; and fishes alone, being 
essentially water-breathing animals, as well as of a decidedly 
inferior grade of organization, never quit that element. But in 
order that a mammal may be adapted to an aquatic existence, 
it must be fashioned more or less in the form of a fish ; an ela- 
borate hand or foot would be useless, and projecting appendages 
injurious. It is therefore piscine in form, covered with a smooth 
skin, and differs from a fish only in the position of the tail, which, 
being horizontal instead of vertical, is an index of its air-breath- 
ing habits. So also an aquatic bird has a smooth covering of 
close-set feathers, an attenuated head, fin-like wings, and feet 
situated so far back as to answer the purpose of a propelling tail 
when in the water; and could we see a Penguin in the act of 
swimming beneath the waves, it would undoubtedly have the 
aspect of a fish. Take, again, the Seals, in which these aquatic 
habits are not so complete as in the Cetaceans, and we find them 
modified in form to be something intermediate between a fish 
and a mammal; while an Otter, which is rather terrestrial than 
aquatic, has its quadrupedal character still less modified: in it 
we find the close-set fur, the depressed form, and the webbed 
feet ; but the feet are not fins, nor is the tail. 
With regard to flying quadrupeds, it is of course more or less 
necessary that the upper extremity should form a wing of some 
kind, which, however different in the homologies of its parts from 
the wing of a bird, must necessarily bear some general resem- 
blance to it in form, A Bat is as purely an aérial animal as is a 
