376 AN INTRODUCTION TO THE STUDY OF FOSSILS 



lion-like cats and giant dogs. The extinction of the majority 

 of these Pleistocene mammals was probably mostly due, either 

 directly or indirectly, to the accompanying glacial conditions. 

 Exceptionally cold waves or unusually prolonged cold seasons 

 even now lead to a temporary diminution in the number of ani- 

 mals in a herd. This may be due to the freezing of the young and 

 other weak members, complete starvation from the deep cover- 

 ing of snow over their food, or merely partial starvation and a 

 consequent inability of the individual females to protect their 

 young from the Carnivora. This reduction of the herd may in 

 turn lead to its complete destruction through the insufficiency 

 in the number of bulls to protect the young ; the reduction in 

 the number of herds leads in turn to inbreeding and to its prob- 

 able accompaniment of infertility. Infertility may likewise 

 often be produced by an increasingly unfavorable climatic 

 environment. The change from a forested condition, such as 

 existed in the Northern Hemisphere north to the Arctic Ocean 

 during the early Pleistocene, to an unforested one, and back 

 again during times of increasing moisture, as happened at 

 least once during the Pleistocene, would cause the diminution 

 of both forest animals, such as the browsing camels, browsing 

 horses, mastodons, elephants, tapirs and deer, and of grazing 

 animals. The remains of the larger Pleistocene animals, now 

 extinct, are nearly always associated with evidences of forests. 

 It has been suggested that a corollary cause in the extinction of 

 the horse from North America may have been some epidemic 

 disease, or diseases, carried by some fly, tick or other parasite- 

 bearing insect ; the multiplication and spread of such insects 

 are increased by the presence of a moist climate. If southern 

 North America accordingly was exceptionally moist during a 

 portion of the Pleistocene, of which there is evidence, some such 

 insect-spread epidemic may have led to the extinction of the 

 horse by the close of the Pleistocene. 



Mammals are divided into the following orders : — 



