204 THE HISTORY OF SCIENCE 



affected by his discovery in Uruguay and Patagonia 

 of the fossil remains of extinct mammals, all the 

 more so because they seemed to bear relationship to 

 particular living species and at the same time to 

 show likeness to other species. The Toxodon (bow- 

 tooth), for example, was a gigantic rodent whose 

 fossil remains were discovered in the same region 

 where Darwin found living the capybara, a rodent 

 as large as a pig ; at the same time the extinct species 

 showed in its structure certain affinities to the Eden- 

 tata (sloths, ant-eaters, armadillos). Other fossils 

 represented gigantic forms distinctly of the edentate 

 order and comparable to the Cape ant-eater and the 

 Great Armadillo (Dasypus gig as). Again, remains 

 were found of a thick-skinned non-ruminant with 

 certain structural likeness to the Camelidae, to which 

 the living species of South American ruminants, the 

 guanacos, belong. 



Why have certain species ceased to exist ? As the 

 individual sickens and dies, so certain species become 

 rare and extinct. Darwin found in Northern Pat- 

 agonia evidence of the Equus curvidens, an extinct 

 species of native American horse. What had caused 

 this species to die out? Imported horses were intro- 

 duced at Buenos Ayres in 1537, and so flourished 

 in the wild state that in 1580 they were found as far 

 south as the Strait of Magellan. Darwin was well 

 fitted by the comprehensiveness of his observations 

 to deal with the various factors of extinction and 

 survival. He studied the species in their natural 

 setting, the habitat, and range, and habits, and food 

 of the different varieties. Traveling for three years 

 and a half north and south on the continent of South 



