34 ESSAY ON CLASSIFICATION 



that the point of view under which I have presented the subject here 

 is the true one; and that in the end the characteristic gradation ex- 

 hibited by the orders of each class will present the most striking 

 correspondence with the character of the succession of the same 

 groups in past ages, and afford another startling proof of the ad- 

 mirable order and gradation in the degrees of complication of the 

 structure of animals, which have been established from the very 

 beginning and maintained throughout all time. 



SECTION IX 

 RANGE OF THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



The surface of the earth being partly formed by water and partly 

 by land, and the organization of all living beings standing in close 

 relation to the one or the other of these mediums, it is in the nature 

 of things that no single species, either of animals or plants, should 

 be uniformly distributed over the whole globe. Yet there are some 

 types of the animal as well as of the vegetable kingdom which are 

 equably distributed over the whole surface of the land, and others 

 which are as widely scattered in the sea, while others are limited to 

 some continent or some ocean, to some particular province, to some 

 lake, nay, to some very limited spot of the earth's surface.^^ 



As far as the primary divisions of animals are concerned, and the 

 nature of the medium to which they are adapted does not interfere, 

 representatives of the four great branches of the animal kingdom are 

 everywhere found together. Radiata, Mollusks, Articulata, and Verte- 

 brata occur together in every part of the ocean, in the Arctics, as well 

 as under the equator, and near the southern pole as far as man has 

 penetrated; every bay, every inlet, every shoal is haunted by them. 

 So universal is this association, not only at present but in all past 

 geological ages, that I consider it as a sufficient reason to expect 



*• The human race affords an example of the wide distribution of a terrestrial type; 

 the Herring and the Mackerel families have an equally wide distribution in the sea. 

 The Mammalia of New Holland show how some families may be limited to one con- 

 tinent; the family of Labyrinthici of the Indian Ocean, how fishes may be circum- 

 scribed in the sea, and that of the Goniodonts of South America in the fresh waters. 

 The Chaca of Lake Baikal is found nowhere else. This is equally true of the Blindfish 

 (Amblyopsis) of the Mammoth Cave, and of the Proteus of the caverns of Carinthia. 



