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 homologous 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 aerial animal as is a 



