ZOOGEOGRAPHY 291 



Mississippi basin to Indiana; Hyla gratiosa from South Carolina to Florida 

 and Mississippi ; Hyla evittata only along the Potomac and York rivers in 

 Virginia and in New Jersey. 



Differences in Continuity of Range. — It has long been noted that cer- 

 tain forms occupy a continuous area while others occur in two or more 

 areas that may be distant from one another. Among the many examples 

 which might be given, the following have been selected. Among the 

 living genera in the family CameHdse (the camels) one, Camelus (the 

 camel and dromedary), occurs in central Asia and northern Africa; 

 and another, Auchenia (the llama and vicuna), is found in western South 

 America. The genus Alligator (the alligators) is composed of two species, 

 one to be found in central China, the other in southeastern United States. 

 These are groups comprising only a few species in each case, yet there are 

 wide gaps in their known ranges. Perhaps more striking is the discon- 

 tinuity sometimes found in the spread of a single species. Thus, the 

 skink Leiolopisma laterale is found in the southeastern United States and in 

 China and certain of the southern islands of the Japanese archipelago. 

 The Central American and Mexican frog, Leptodactylus albilabris, occurs 

 in the West Indies (on the Virgin Islands, Vieques and Porto Rico) as 

 well as on the mainland. 



It is well established that discontinuous distribution occurs more 

 frequently and is more pronounced in the larger taxonomic categories, 

 that is, it is more common and the gaps are wider in genera than in species, 

 in families than in genera, in orders than in families, etc. Indeed it is so 

 rare in species as to make it a matter of doubt in most instances whether 

 discontinuity of a species range may not be due to convergent develop- 

 ment; that is, it may be that the forms in widely separated areas which 

 look so much alike that they are regarded as the same species are not 

 really so closely related, but have come to resemble one another by an 

 independent evolution towards the same goal. Discontinuity in the 

 range of a species may also be due to transportation by man, or to acci- 

 dental dispersion by nature, such as transportation to islands by means 

 of floating vegetation. 



A fact in regard to discontinuous distribution that is constantly re- 

 ceiving confirmation, particularly from paleontology, is that groups now 

 interrupted in distribution were at one time continuous. Thus the pres- 

 ence of fossils shows that the camels, now in two or three regions separated 

 by thousands of miles, formerly occurred more widely in Asia and also in 

 North America; that tapirs have occurred in Europe, Asia and North 

 America, although now found only in the Malay Archipelago and tropical 

 America; and that a near relative of the alligators formerly ranged widely 

 in North America and Europe. So much evidence of this kind has been 

 obtained that it is now generall}'^ concluded that the distribution of groups 

 with interrupted distribution was at one time continuous, except where 



