io62 PRINCIPLES OF STRATIGRAPHY 



tion between eastern Asia and Australia is also indicated by the 

 distribution of the snails (Helix). This bridge appears to have 

 existed before the Upper Cretacic. The connection between Africa 

 and India demanded by the distribution of the fresh water decapods 

 seems to have been in mid-Cretacic time or earlier. Other facts 

 show that this union continued through Eocenic time. The con- 

 nection betv/een New 'Zealand and Australia is believed to have 

 been by way of New Caledonia and New Guinea, and belongs to 

 pre-Eocenic time ; while that between Australia, New Zealand and 

 South America, generally assumed by students of zoogeography, is 

 believed by Ortmann to have been across the Pole, and to have con- 

 tinued to the end of Alesozoic time. There are indications that 

 the West Indies, Central America, and the northern margin of 

 South America formed the "Antillean continent" during Jurassic 

 and Cretacic time, a remnant of and successor to Appalachia; after 

 this was broken up, the northern remnant, consisting of the Greater 

 Antilles and parts of present Central America, probably remained 

 a unit up to the Eocenic, after which it was dismembered, to be 

 once more established in Pleistocenic time, and finally destroyed 

 in the present. Einally, the connection between South America and 

 Africa is believed to have existed in Jurassic and early Cretacic 

 time, but was severed in subsequent periods. 



Terrestrial animals, especially mammals, are even better indices 

 of former land connections than the fresh water animals cited, 

 inasmuch as even moderate dividing straits form absolute barriers 

 to the majority of types. For an extensive account of this subject 

 the student is referred to Lydekker's "Geographical Distribution 

 of Mammals." 



Relicts. 



Relict Faunas and Lakes. The occurrence of halo-limnic 

 organisms or animals or plants normally of marine type in conti- 

 nental seas or lakes has been a matter of great scientific interest 

 ever since Loven, in i860, announced the presence of Crustacea 

 closely related to marine types in the great fresh water lakes of 

 Sweden, and showed by the geological structure of the region that 

 these organisms entered the lakes at a time when they were con- 

 nected with the sea. They, therefore, constituted the relics of a 

 former marine fauna, which had mostly Ijecome extinct by the grad- 

 ual freshening of the waters of the lakes after the connection with 

 the sea was broken. Similar left-over marine faunas or relicts 

 were discovered in many other continental seas, both fresh and 



