28 MAMMALIAN DENTITION 



the scene of the extermination of vast herds of cattle or sheep 

 in a blizzard; the camels which wander half wild amid the 

 ruins of Asiatic towns are probably the descendants of those 

 domesticated animals surviving the dust storm in which their 

 owners perished. In such several ways are preserved the relics 

 of ancient faunas but rarely are they undisturbed; the action 

 of rivers and still more that of glaciers has cleaned from the 

 slate of history many of the poor remnants that remained and 

 not once but many times has the slate been cleaned since Mam- 

 mals first appeared. 



In the more northern part of Europe, Asia and America Ave 

 must not expect to find deposits of animal remains in such 

 profusion as in the regions further south which have not been 

 exposed to glacier action. The southern continents of Africa 

 and South America have been stated to be the great primary 

 centers of mammalian distribution on account of the quanti- 

 ties of fossils preserved there in deposits of all geological epochs 

 since the first Mammals appeared. They may have been sec- 

 ondary centers but it is altogether doubtful if Mammals origi- 

 nated upon either one of them. 



The relation of climate to evolution and to dispersal of ani- 

 mals is most important. If we note the periods of alternate 

 cold and warmth we find that it is in the former alone that 

 great strides are taken by animal life. In the accompanying 

 table (Fig. 2) it will be observed that it was during the glacial 

 phase of the Devonian period that land animals in the form of 

 Amphibia first appeared. Again during the cold stage of the 

 Permian we note the rise of Reptiles and probably the first 

 beginning of the mammal-like creatures. On the other hand 

 during the warm moist Jurassic epoch we find the Amphibia 

 which have by this time evolved into a very numerous group 

 decadent and already disappearing. The Reptiles, a vast and 

 miscellaneous host, some of them stupendous in size, begin to 

 disappear during the Cretaceous period which included a warm 

 moist stage sandwiched between two glacial phases. During 

 this epoch we note also the rise of archaic Mammals which be- 



