CiiAP. T. SIZE AND THE ELEMENTS. 49 



hiis not yet assumed any of those more determined characteristics which distinguish 

 the full-grown aniiual or tlie 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 tills 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, AKD THE MEDIUMS IN WHICH THEY LIVE. 



It has just been remarked, that animals of different tj'jies, even when hving 

 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 owti 

 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 

 KeptUes. 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. Tlie same relation is observed in the different fixmilies of Insects which 

 number aquatic and terrestrial species. It is further remarkable, that among aquatic 

 animals, the fresh water types are inferior in size to the marine ones ; the marine 

 Turtles 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 generally lai'ger than the more terrestrial 

 Clemmys or the Cistudo. The class of Fishes has its largest representatives in the 

 sea ; fresh water fishes are on the whole dwarfs, in comparison to their marine 

 relatives, and the largest of them, our Sturgeons and Salmons, go to the sea. The 

 same relations obtain among Crustacea ; to be satisfied of the fact, we need only 

 compare oiu- Crawfishes with the Lobsters, our Apus with Limulus, etc. Among 



* Geoffroy St. IIii.aike, (Isid.,) Recherches humaines, Paris, 1831, 4to. — See also my paper 

 zoologicpies et physiologiqucs sur Ics variations upon the Natural Relations between Animals and the 



lie la taillu elioz les Animaux ct dans les races Elements, etc., quoted above, p. 32. 



