184 ESSAY ON CLASSIFICATION. 



ages. Among the Gasteropoda, 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 Echinoderrns, the Holo- 

 thurians and Spatangoids, are most diversified within the 

 tropics, while the Echini, Starfishes, and Ophiurse 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 Actinoid Polypi, are entirely tropical ; while the 

 highest Halcyonoids, Eenilla, Veretillum, and Pennatula, 

 extend to the tropics and the temperate zone. 



Another interesting relation between the geographical 

 distribution 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 representa- 

 tives of the oldest geological periods : Pentacrinus is not 

 found before the Lias ; among Cephalopods we find the 

 Nautilus, but nothing like Orthoceras ; Limulus, but no- 

 thing 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 pecu- 

 liar structures, are so strangely limited in their range ; 

 and yet, even this shows how closely the geographical dis- 

 tribution of animals is connected with their structure. 

 Why New Holland should have no Monkeys, no Carni- 

 vora, no Ruminants, no Pachyderms, no Edentata, is not 

 to be explained ; but that this is the case, every zoologist 

 knows, and is further aware that the Marsupials 1 of that 



1 See Sect. 11. 



