74 H. D. MACGINITIE 



forth along the Gulf Coast from the Appalachians to the mountains 

 of eastern Mexico (Sharp, 1950, pp. 316-318). 



The world climate has undergone striking fluctuations since the 

 retreat of the last continental glacier, without physiographic changes 

 of any consequence. A gradual warming culminated in the "climatic 

 optimum" (about 6,000 years ago), followed by a return to cooler 

 conditions. Now a warming trend seems to have been renewed. 

 Prehistoric sites of human habitation in the western states show 

 clear evidence of great climatic instability since the retreat of the 

 glaciers (Malin, 1957). Occupance layers are separated by thick 

 layers of wind-blown dust that indicate intervals when the sites 

 were uninhabited and when the vegetational cover was at a mini- 

 mum. There are historical accounts of severe dust storms in the 

 Plains states long before modern settlement and the "plowing of 

 the plains." One of the latest effects of climatic changes is the 

 spread of the tropical deserts from western India across the 

 Mediterranean and northern Africa, and in our own Southwest 

 (Wadia, 1955). 



Attention is called to two stimulating papers dealing with the 

 present apparently erratic distribution of certain vertebrate ani- 

 mals. In dealing with the biogeographical problems concerning some 

 American genera of salamanders, Lowe (1950) has plausibly ex- 

 plained the puzzling distribution of these animals by a considera- 

 tion of late Tertiary and Pleistocene changes. Smith (1957) has 

 treated some unusual problems concerning the distribution of mam- 

 mals in the north-central states. Almost any type of erratic distri- 

 bution and almost any inconsistency of distribution appear possible 

 when the effects of climatic changes in the later Tertiary and Pleis- 

 tocene are considered. 



The world climate has been comparatively stable throughout the 

 ages, but subject, at times, to the most amazing variations. In 

 emphasizing certain aspects of past climates the writer has hoped to 

 furnish some hints concerning the remarkable effects of climatic 

 changes on the evolution and distribution of living plants and 

 animals. 



Space does not permit a critical presentation of all the different 

 theories and ideas as to the causes of world climatic changes (Flint, 

 1957; Willett, 1953). However, in considering these causes it is al- 

 ways necessary to go back to the two fundamental controls of world 



