396 P. S. MARTIN 



discoveries in both prehistory and paleontology have enriched 

 considerably our knowledge of late Pleistocene fauna. Radiocarbon 

 dates (Fig. 4, Table II) confirm the fact, evident to Darwin and 

 Lyell, that extinction was mainly a Postglacial event. South of 

 Alaska there is abundant proof that the time of maximum glaciation 

 preceded most New World extinction. 



Despite these refinements, the extinction of large mammals in 

 continental North and South America and of small mammals in the 

 West Indies has been a major unsolved problem, one certainly not 

 inappropriate in a symposium dedicated to such matters. Until the 

 cause or causes of extinction are understood, biogeographic and 

 ecological interpretations based on the assumption that all fossil 

 mammalian records are of paleoclimatic significance may be overly 

 bold, if not entirely erroneous. Specifically, I would question paleo- 

 climatic deductions based on fossil records of Marmota and Cervus 

 in northeastern Mexico, Erethizon (porcupine) and Hydrochoerus 

 (capybara) in the Melbourne beds of Florida, "musk-ox-like" 

 genera in New Mexico and Mexico, and Tapirus in Arizona (Haury 

 et al., 1950) and Pennsylvania. Strict application of the uniformi- 

 tarian doctrine is to be avoided in each case ; no responsible ecologist 

 would insist that modern tapir habitat, Tropical Rainforest and 

 Cloud Forest, extended into southern Arizona or eastern Pennsyl- 

 vania in the late Pleistocene. 



As Darwin stressed in The Origin of Species, extinction is the 

 inevitable consequence of evolution and in itself will occasion no 

 surprise. Through the Cenozoic equid genera disappear; Hyra- 

 cotherium and others in the Eocene; Mesohippiis in the Oligocene; 

 Miohippus, Parahippus, and others in the Miocene; Calippus, 

 Hipparion, etc., in the Pliocene; and Nannippus and Plesippiis in 

 the early Pleistocene. They represent a record of replacement by 

 morphologically modified and adaptively improved types of horses. 

 In the late Pleistocene the extinction of North American Equus 

 and South American Equus and Hippidium is not an equivalent 

 event for it constitutes extinction without replacement. For perhaps 

 3,000 to 6,000 years in the Americas the horse was absent. Follow- 

 ing post-Columbian reintroduction, feral horses reoccupied grass- 

 land habitats with unseeming haste. Darwin (1855, p. 299) reported 

 that they spread from Buenos Aires to the Straits of Magellan, 1,300 

 miles, in 43 years. 



