VERTEBRATES — CLARK 301 



regions. Volcanic activity would also produce a large amount of 

 atmospheric dust. 



Of interest in regard to the latter is the information on the eruption 

 of Krakatoa between Sumatra and Java on May 26-28, 1883. In this 

 eruption the ejected column of stones, ashes, and dust was estimated 

 to have reached a height of 17 miles or more. The finer particles were 

 diffused over a large part of the earth, and were carried over North 

 and South America, Europe, Asia, South Africa, and Australia. In 

 the Old World they spread from Scandinavia to the Cape of Good 

 Hope. And this is the record of only a single isolated eruption. 



With increase in land areas and intermittent volcanic eruptions 

 the illumination of the surface of the earth would be considerably al- 

 tered. Whereas the extensive inundation of the land areas during 

 the Cretaceous would presumably clear the atmosphere, giving rise to 

 conditions especially favorable to reptiles, increase in land areas, 

 especially in connection with volcanic activity, would make conditions 

 unfavorable for reptiles while at the same time permitting the rapid 

 increase and diversification of mammals. 



In their present distribution the land reptiles fall into two main 

 groups. The strictly terrestrial forms, such as the lizards and most 

 snakes, follow largely the distributional pattern of the mammals ex- 

 cept that they do not range so far north or south and, possibly because 

 of their greater age, there are among them more striking cases of dis- 

 continuous distribution resulting from extirpation over a large por- 

 tion of the original range. In the mammals examples of discon- 

 tinuous distribution are the camels, originally North American but 

 now represented only by two wild species in South America, two 

 domesticated species in South America, and two in Asia, one of which 

 has been introduced into Africa ; the tapirs, once widespread but now 

 restricted to tropical America and the Malay Peninsula, Sumatra, and 

 Borneo ; and the fresh- water dolphins of South America, India, and 

 China. In the Pleistocene many mammals spread from Asia into 

 North America, but apparently the connection between these conti- 

 nents was too far north to serve as a highway for any reptiles. Some 

 examples of discontinuous distribution in the reptiles are : The Gila 

 monsters (Helodermatidae), Texas, Arizona, Mexico, and Borneo; 

 the large herbivorous iguanas, tropical America, Madagascar, and 

 Fiji; the pythons, Old World and western Mexico; and the burrowing 

 Amphisbaena, South America and Africa. The true land tortoises 

 (Testudo) are found in all tropical and warm temperate regions ex- 

 cept Australia. 



The amphibious reptiles, the crocodilians and fresh-water turtles, 

 present a more generalized picture owing to the fact that aquatic 

 habitats are much less variable than terrestrial, the chief differential 



