FORMER LAND CONNECTIONS. 319 



" In the case of the above-mentioned Arthropods, no reason 

 can be assigned for their extermination elsewhere in the tropics, 

 if they are the only extant representatives of genera, formerly 

 widely distributed in the Northern Hemisphere." 



Mr. Pocock also presented evidence in favour of a former 

 trans-Atlantic connection from the distribution of the following 

 groups of mammals : — Sirenia ; the genus Trichechus ; the 

 Primates; and the Hystricomorph Kodents. The Manatees are 

 restricted to the rivers and estuaries debouching into the Atlantic 

 on the American and on the African sides. These animals do 

 not venture out to sea, and no extinct representatives of the 

 genus appear to be known from European or North American 

 deposits to support the theory of its former extension into 

 northern latitudes. 



" The headquarters of the Hystricomoi-pha at the present 

 time are South America, where they date back to the Upper 

 Miocene. The only North American representative is the tree- 

 porcupine, a late immigrant from South America. No extinct 

 representatives have been found in early or mid-Tertiary strata 

 in North America. But, in the Old World, alleged representatives 

 of the sub-order, referred to the family Theridomyidae, occur in 

 Eocene and Oligocene deposits in Europe, and at the present time 

 several genera of Octodontidae occur in Africa, and the 

 Hystricidae range from Africa through Southern Asia to Borneo. 

 Until evidence for the existence of the group in early and mid- 

 Tertiary or Cretaceous times in North America is forthcoming, 

 it cannot reasonably be claimed that the South American forms 

 are descendants from ancestors from the North." 



" The past and present distribution of monkeys is tolerably 

 similar to that of the Hystricomorph Eodents." Mr. Pocock did 

 not think it probable that the resemblances between the Old and 

 New World monkeys could be due to convergent descent from 

 Lemuroids of the Old and New Worlds respectivel3^ 



Facts and Theories ix Opposition to a Trans-Atlantic 

 Hypothesis. 



(.4.) Keferring solely to the mammalian data, Dr. C. W. 

 Andrews® remarked that if a land-bridge had existed between 

 Africa and South America in Tertiary times, one would expect 

 a more extensive mingling of faunas than had actual^ taken 

 place. Even in the Eocene, both continents must have had a 

 varied mammalian fauna, yet it is only claimed that the Primates, 

 the Hystricomorph Rodents, and perhaps some Insectivora, 

 crossed from Africa to South America, no interchange in the 

 opposite direction being known. The Primate evidence he rejected 

 as being due to convergent evolution : the Hystricomorph Rodents, 

 being all small animals, seem to have been very abundant, so 

 that like rats and mice of to-day, they would be specially liable 

 to accidental transport, and no land-bridge theory need be raised. 



Professor Osborn*^ also considered that the geographical distri- 

 bution of land mammals does not favour an Atlantis hypothesis, 



