50 THE PRESENT. 



or to means of distribution that prevailed during former 

 geological epochs, but which ceased to exist when sea and 

 land received their present relations. And this brings us 

 to remark 011 what are termed by zoologists the law of 

 identity and the law of representation ; that is, that different 

 regions, though not peopled by identical species, may be 

 peopled by animals which perform analogous functions, and 

 represent them, as it were, in the great plan of vital eco- 

 nomy. Thus, the ostrich of Africa is represented in South 

 America by its congener the rhea ; the jaguar and puma of 

 the New World represent the tiger and lion of the Old; 

 the camel of Arabia finds its analogue in the llama of Peru ; 

 and similar functions are at once discharged by the gavial 

 of the Ganges, the crocodile of the Nile, and the alligator of 

 the Amazon. Over and above these physical relationships 

 there is also that which has reference to the size of the 

 animal, and the element in which it is destined to live. 

 As a general rule, and each within its own order or family, 

 the aquatic members are larger than the terrestrial; the 

 amphibious bulkier than those that are strictly terrestrial; 

 the marine superior in size to those of fresh- water habitat ; 

 and the terrestrial more massive than the arboreal. Ad- 

 mitting these relations, and reasoning from the present to 

 the past, the comparative bulk of organic remains may often 

 become an index to external conditions of life, and throw 

 light over the investigations of the palaeontologist, when 

 other indications are uncertain and obscure. 



Besides these distinctions and restrictions imposed on 

 vitality by external conditions, there are those connected 

 with the functions they have to perform in the economy of 

 nature. Some, for instance, are fitted to live on a purely 

 vegetable diet, others to prey on the flesh of other creatures ; 

 gome are constructed so as to feed only on seeds and grains, 

 others to prey solely on insects ; many earn their subsist- 



