FUNDAMENTAL RELATIONS OF ANIMALS 121 



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 mountains than 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 Brachyura, are most 

 numerous in the torrid zone; but Dana has shown, what was not 

 at all expected, that they nevertheless reach their highest perfec- 

 tion in the middle temperate regions. The Anomoura and Macroura, 

 on the contrary, are nearly equally divided between the torrid and 

 temperate zones; while the lower Tetradecapods are far more nu- 

 merous in extra tropical latitudes than in the tropical. The Cepha- 

 lopods are most diversified within the tropics; yet the Nautilus is 

 a reminiscence of past ages. Among Gasteropods, the Stromboids 

 belong to the tropics; but among the lamellibranchiate Acephala, 

 the Naiades, which seem to me to stand very high in their class, have 

 their greatest development in the fresh waters of North America. 

 The highest Echinoderns, the Holothurians and Spatangoids are 

 most diversified within the tropics, while Echini, Starfishes, and 

 Ophiuras extend to the arctics. The presence of Pentacrinus in the 

 West Indies has undoubtedly reference to the prevalence of Cri- 

 noids in past ages. The Madrepores, the highest among the Actinoid 

 Polypi, are entirely tropical, while the highest Halcyonoids, the 

 Renilla, Veretillum, and Pennatula, extend to the tropics and the 

 temperate zone. 



Another interesting relation between the geographical distribu- 

 tion of animals and their representatives in past ages is the absence 

 of embryonic types in the warm regions. We find in the torrid zone 

 no true representatives of the oldest geological periods; Pentacrinus 



