30 MAMMALIAN DENTITION 



came especially numerous during the warm moist Eocene and 

 then gradually gave place to modern types of Mammals which 

 evolved during the cold Pleistocene known usually as The 

 Glacial Period. 



From the foregoing it is apparent that periods of cold have 

 a stimulating influence upon animal life whereas warm equable 

 climates by their enervation result in deterioration and final 

 extinction of the type. Extermination is preceded by increase 

 in size and by lethargy; this is natural since it is easier to ob- 

 tain food as a rule in a warm climate and life is much less 

 strenuous. In the colder environment the animal must be alert 

 and vigorous to maintain its existence. In some obscure man- 

 ner climate has a marked influence upon the evolution of dif- 

 ferent types in adaptation to the ever-changing environment 

 for we note the successive dominant types Amphibia, Reptilia 

 and Mammalia. 



Intimately associated with the environment is the dispersal 

 of animals. It stands to reason that a creature which is not 

 capable of changing its life habits to suit its changing circum- 

 stances must migrate when the climate changes in order to fol- 

 low the receding warmth and maintain an unchanged environ- 

 ment. Paleontology shows us that these migrations have 

 usually been from north to south exactly as we would expect 

 since we have observed that it is the cold phase which stimulates 

 progressive evolution. 



Before going further we should differentiate clearly between 

 the terms primitive, generalized, advanced, and specialized as 

 applied to Mammals. A primitive animal is one which has re- 

 tained many of its ancestral or archaic characters; it may be 

 very specialized but it cannot be generalized nor can it be 

 advanced. A generalized animal may be primitive and usually 

 is in some respects; it is not specialized because it retains the 

 ability to change its life habits with changing environment. 

 An advanced animal may be generalized or it may be specialized 

 but it will not be primitive; it has changed greatly from the 

 ancestral type. A specialized animal may be primitive or ad- 



