Ciup. I. GRADATION, GROWTH, SUCCESSION, DISTRIBUTION. 121 



tive standing in tliuir respective classes, and to the order of their succession in 

 past geological ages, and more indirectly, also, to their embryonic growth. 



Almost every class has its tropical families, and these stand generally highest 

 in their respective classes; or, when the contrary is the case, when they stand 

 evidently upon a lower level, there is some prominent relation between them and 

 the pi'evailing types of past ages. The class of Mammalia affords striking examples 

 of these two kinds of connection. In the first place, the Quadrumana, which, next 

 to Man, stand highest in their class, are all tropical animals ; and it is worthy of 

 remark, that the two highest types of Anthropoid Monkeys, the Orangs of Asia and 

 the Chimpanzees of Western Africa bear, in the coloration of their skin, an addi- 

 tional similarity to the races of Man inhalnting the same regions, the Orangs being 

 yellowish red, as the Malays, and the Chimpanzee blackish, as the Negroes. The 

 Pachyderms, on the contrary, stand low in their class, though chiefly tropical ; but 

 they constitute a group of animals prominent among the earliest representatives of 

 that class in past ages. Among Chiroptera, the larger frugivorous representatives are 

 essentially tropical; the more omnivorous, on the contrary, occur everywhere. Among 

 Camivora, the largest, most powerful, and also highest types, the Digitigrade, prevail 

 m 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 Ruminants, we find the Giraffe 

 and the Camels in the warmer zones, the others everyAvhere. In the class of Birds 

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

 by far the largest representation 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 Reptiles, the Crocodilians are entirely tropical; the largest 

 land Turtles are also only found in the tropics, and the aquatic representatives of 

 this order, which are evidently inferior to their land kindred, extend much further 

 north. The Rattlesnakes and Vipers extend further north and higher up the moun- 

 tains thiin the Boas and the common harmless snakes. The same is true of Sala- 

 manders 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 Crustacea the highest order, the Bra- 

 chyoura, 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.^ The Anomoura and Macroura, on the contrary, ai-e nearly 

 equally divided between the torrid and temperate zones; while the lower Tetrade- 

 capods are far more numerous in extra tropical latitudes than in the tropical. The 



* Dana, Crustacea, p. 1501. 

 IG 



