GRADATION, GROWTH, SUCCESSION, DISTRIBUTION. 183 



powerful, and also highest types, the Digitigrade, prevail 

 in the tropics ; while among the Plantigrades, the most 

 powerful, the Bears, belong to the temperate and to the 

 arctic zone, and the lowest, the Pinnate, are marine species 

 of the temperate and arctic seas. Among the Euminants 

 we find the Giraffe and the Camels in the wanner zones, 

 the others everywhere. In the class of Birds, the gradation 

 is not so obvious as in other classes, and yet the aquatic 

 types form by far the most numerous representatives of 

 this class in temperate and cold regions, and are almost 

 the only ones found in the arctic, while the higher, land 

 birds prevail in the warm regions. Among the Reptiles, the 

 Crocodilians are entirely tropical ; the largest land Turtles 

 are also only found in the tropics, and the aquatic repre- 

 sentatives of this order, which are evidentlv inferior to 



it 



their land-kindred, extend much further north. The Rat- 

 tlesnakes and Vipers extend further north and higher up 

 the mountains than the Boas and the common harmless 

 snakes. The same is true of the Salamanders and Tritons. 

 The Sharks and Skates are most diversified in the tropics. 

 It is also within the tropics that the most brilliant diurnal 

 Lepidoptera are found, and this is the highest order of 

 Insects. Among the Crustacea the highest order, the Bra- 

 chyora, are most numerous in the torrid zone ; but 

 Dana has shown, what was not at all expected, that they 

 nevertheless reach their highest perfection in the middle 

 temperate regions. 1 The Anornoura and Macroura, on the 

 contrary, are nearly equally divided between the torrid 

 and temperate zones ; while the lower Tetradecapods are 

 far more numerous in extra-tropical latitudes than in the 

 tropical. The Cephalopoda are most diversified within 

 the tropics ; yet the Nautilus is a reminiscence of past 



1 DANA, Crustacea, p. 1501. 



