122 ESSAY ON CLASSIFICATION. Part I. 



Ceplialopods are most diversified within the tropics; yet the Nautilus is a reminis- 

 cence 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 Echinoderms, the Holothurians and Spatangoids are most diver- 

 sified within the tropics, while Echini, Starfishes, and Ophiura) extend to the arctics. 

 The presence of Pentacrinus in the West Indies has undoubtedly reference to the 

 prevalence of Crinoids in past ages. The Madrepores, the highest among the Acti- 

 noid Polypi, are entirely tropical, while the highest Halcyonoids, the Renilla, Vere- 

 tillum, and Pennatula, extend to the tropics and the temperate zone. 



Another interesting relation between the geographical distriljution 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 geo- 

 logical periods ; Pentacrinus is not found before the Lias ; among Cephalopods we 

 find the Nautilus, but nothing like Orthoceras; Limulus, but nothing like Trilobites. 



This study of the relations between the geographical distribution of animals, and 

 their relative standing, is rendered more difficult, and in many respects obscure, by 

 the circumstance that entire types, characterized by peculiar structures, are so 

 strangely limited in their range; and yet, even this shows how closely the geographi- 

 cal distribution of animals is connected with their structure. Why New Holland 

 should have no Monkeys, no Carnivora, no Ruminants, no Pachyderms, no Edentata, 

 is not to be explained ; but tliat this is the case, every zoologist knows, and is 

 further aware, that the Marsupials^ of that continental island represent, as it were, 

 the other orders of Mammalia, under their special structural modifications. New 

 Holland appears thus as a continent with the characters of an older geological age. 

 No one can fixil, therefore, to perceive of how great an interest for Classification 

 will be a more extensive knowledge of the geographical distribution of animals in 

 general, and of the structural peculiarities exhibited by locahzed types. 



SECTION XXIX. 



MUTUAJL DEPEJTDENCE OF THE ANIMAI, AND VEGETABLE KmGDOMS. 



Though it had long been known, by the experiments of De Saussure, that the 

 breathing process of animals and plants are very different, and that while the for- 



> See Sect. 11. 



