56 ESSAY ON CLASSIFICATION 



velopment, at a period when the germ they contain, has not yet 

 assumed any of those more determined characteristics which distin- 

 guish the full-grown animal or the perfect plant? Do physicists know 

 a law of the material world which presents any such analogy to these 

 phenomena, that it could be considered as accounting for them? 



In this connection it should be further remembered that these 

 cycles of size characteristic of different families are entirely different 

 for animals of different types, though living together under identical 

 circumstances. 



SECTION XIV 



RELATIONS BETWEEN THE SIZE OF ANIMALS AND THE 

 MEDIUMS IN WHICH THEY LIVE 



It has just been remarked, that animals of different types, even 

 when living together, are framed in structures of different size. Yet 

 life is so closely combined with the elements of nature, that each type 

 shows decided relations, within its own limits, to these elements as 

 far as size is concerned^* The aquatic Mammalia as a whole are 

 larger than the terrestrial ones; so are the aquatic Birds and the 

 aquatic Reptiles. In families which are essentially terrestrial the 

 species which take to the water are generally larger than those which 

 remain permanently terrestrial, as for instance, the Polar Bear, the 

 Beaver, the Coypu, and the Capivara, Among the different families 

 of aquatic Birds those of their representatives which are more ter- 

 restrial in their habits are generally smaller than those which live 

 more permanently in water. The same relation is observed in the dif- 

 ferent families of Insects which number aquatic and terrestrial spe- 

 cies. It is further remarkable that among aquatic animals the fresh- 

 water types are inferior in size to the marine ones; the marine Tur- 

 tles are all larger than the largest inhabitants of our rivers and ponds, 

 the more aquatic Trionyx larger than the Emyds, and among these 

 the more aquatic Chelydra larger than the true Emys, and these 



''* Isidore Geoffroy St. Hilaire, Recherches zoologiques et physiologiques siir les varia- 

 tions de la taille chez les animaux et dans les races humaines (Paris, 1831). See also 

 my paper upon the "Natural Relations between Animals and the Elements . . . ," 

 American Journal of Science, IX (2d ser., 1850), 369-394. 



