ZOOGEOGRAPHY 365 



temperate (8,000-9,000 to 11,000-12,000 feet) ; paramo or puna (11,000-12,000 

 to lower snow limit at 15,000-16,000 feet) . Chapman found relatively abrupt 

 changes in bird species between zones and there is no reason to suppose that 

 the pselaphids would not be even more limited. So far definite neotropical 

 altitude records show pselaphids to be in the first three zones but no data are 

 available to show increasing relationship with the nearctic fauna with in- 

 creasing elevation. This should be interesting to work out since (Humboldt, 

 1850) from pole to equator mean temperature increases about one degree 

 Fahrenheit with each degree latitude, and ascent of the Andes (Outright, 

 1940) shows a decrease in the mean air temperature of about one degree 

 Fahrenheit for each 300 feet — that is, 300 feet elevation in the Andes is 

 equivalent to about 67 miles of latitude. The importance of mountains, both as 

 barriers to dispersal and as dispersal highways, is summarized in general terms 

 in Hesse, Allee and Schmidt (1937) and is too well known to prolong this dis- 

 cussion without specific information. 



The second intra-tropical influence relates more especially to the Antillean 

 area and is concerned with the possibility of dispersal of pselaphids by natural 

 rafts or floating masses of vegetation (Wallace, 1880) . Any such debris from 

 the mouths of the Amazon, Essequibo, or Orinoco rivers might move north- 

 ward on the arm of the South Equatorial Current that develops north of 

 Pernambuco, passing between Trinidad and Martinique, and if it withstood the 

 passage might secondarily colonize the Lesser Antilles. Again, drift would tend 

 to move from the Lesser Antilles west by north to the Yucatan coast. Finally, 

 drift from between Nicaragua to Yucatan would tend to eventually attain the 

 Gulf Stream and pass through the Straits of Florida. These remote possibilities 

 should be kept in mind since rafts have been sited far at sea, and a few minute 

 oribatid mites in a handful of leaf mold could support a pselaphid population 

 for a long time. 



To continue the analysis, some remarks are desirable on the horizontal 

 range of pselaphids. In the first place the great majority of the 895 neotropical 

 species listed in this paper are valid, that is, contain few synonyms which 

 might give a necessarily imperfect idea of species range. The sound condition 

 of this taxonomy is due to two chief reasons. The majority of the species were 

 described prior to 1904 by four Europeans, Sharp, Schaufuss, Reitter and 

 Raffray ; Raffray was able to examine practically all of the Reitter and Schau- 

 fuss types, and at least some of the Sharp types, and hence could discover 

 synonyms of his own and others. This placed the pselaphids of the neotropics 

 on a fairly firm, if limited foundation, upon which the modern school could 

 build. 



The debt we owe to these four workers is easily measured by their taxo- 

 nomic activity. In the following table, it will be seen that eighteen men have 

 described 897 species in the Americas south of the United States (this figure 

 includes two northern Mexican species, not properly neotropical). Of these 

 the first four, mentioned above, have described 751 species, while Raffray has 

 described nearly half of the entire number! 



