xiv EDITOR'S INTRODUCTION 



in parts of Europe and North America resulted in the successive 

 transformation of once tropical regions to temperate zones, pine 

 forests, barren tundras, and slowly moving masses of ice. As the ice 

 advanced, some arctic forms moved southward and established them- 

 selves in tropical regions. Some animals returned to their former 

 homes with the retreat of the ice and the revival of warmth, but 

 others remained permanently in their new locales. Animals and 

 plants in the path of glacial advance either moved further south, 

 were extinguished by the cold, or adapted to the new conditions of 

 life. These changes in zoological geography resulted in the con- 

 temporary admixture of arctic forms with those of temperate and 

 tropical regions. Glaciation and geographic changes related to it 

 thus explained the dispersal of flora and fauna from their original 

 habitat, and the consequent operations of natural selection in pre- 

 serving successful variations that suited new conditions of life. 



Agassiz, however, found it impossible to interpret the results of his 

 research in such a fashion as to provide a positive role for "physical 

 agencies" in natural history. Once again, offering substantiation for 

 Cuvier's interpretations, he saw the glaciers as "God's great plough," 

 destructive forces that yet signified supernatural intervention. To 

 Agassiz the glacial period was a magnificent albeit chilly demonstra- 

 tion of the power of the Deity to cause great catastrophes, events 

 which had wiped out life in previous epochs, after which it was 

 created anew by divine action. Therefore, it was impossible for any 

 genetic relationship to exist between past and present forms, so 

 firm was his opposition to concepts of unity of origin and common 

 development in nature. As Agassiz characterized recent natural his- 

 tory. 



There is ... a complete break between the present creation and those [crea- 

 tions] which precede it; if the living species of our times resemble those buried 

 in the levels of the earth, so as to be mistaken for them, it cannot be said that 

 they have descended in direct line of progeniture, or what is the same thing, 

 that they are identical species.3 



The early achievements of his career established Agassiz as a wide- 

 ranging scientist, capable of generalizing about nature as a result of 

 distinguished special studies in geology, paleontology, and ichthy- 



* Quoted in Jules Marcou, Life, Letters, and Works of Louis Agassiz (2 vols., New 

 York, 1896), I, 107. 



