70 ESSAY ON CLASSIFICATION. 



SECTION XIII. 



RELATION BETWEEN THE SIZE OF ANIMALS AND THEIR 



STRUCTURE. 



The relation between the size and the structure of ani- 

 mals has been very little investigated, though even the most 

 superficial survey of the animal kingdom may satisfy any 

 one, that there is a decided relation between size and 

 structure among them. Not that I mean to assert that 

 size and structure form parallel series, or that all animals 

 of one branch, or even those of the same class or the same 

 order, agree very closely with one another in reference to 

 size. This element of their organization is not denned 

 within those limits, though the Vertebrata, as a whole, 

 are larger than Articulata, Mollusks, or Radiata; though 

 Mammalia are larger than Birds, Crustacea larger than 

 Insects ; though Cetacea are larger than Herbivora, these 

 larger than Carnivora, etc. The true limit in the organiza- 

 tion of animals, within which size acquires a real import- 

 ance, is that of families, that is, groups which are essen- 

 tially distinguished by their form ; as if form and size 

 were correlative as far as the structure of animals is 

 concerned. The representatives of natural families are, 

 indeed, closely similar in that respect. The greatest 

 differences within these limits are hardly anywhere 

 as much as ten to one, and frequently not more 

 than as two to one. A few examples, selected from 

 among the most natural families, will show this. Omit- 

 ting mankind, on account of the objections which might 

 be made against the idea that it embraces any original 

 diversity, let us consider the different families of Mon- 

 keys, of Bats, of Insectivores, of Carnivores, of Rodents, of 

 Pachyderms, of Ruminants, etc, ; among Birds, the Vul- 



