I20 PERIOD IV. 



shown to be necessary. When Darwin and Wallac< 

 pointed out how immensely important is the bearing- 

 upon present distribution, not only of the physical 

 history of the gfreat continents, but also of their bio- 

 logical history, and in particular of the interminable 

 conflicts of races of which they have been the scene, 

 naturalists began to perceive how inadequate are hori- 

 zontal and vertical isothermal zones to explain all the 

 striking facts of distribution, whether of plants or 

 animals (see infra,, p 129). 



Premonitions of Biological Evolution. 



The eighteenth century had done much to impress 

 the minds of men with an orderly development in sun 

 and planets (Kant and Laplace), in the institutions of 

 human societies (Montesquieu), and in the moral aspira- 

 tions of mankind (Lessing). Many bold attempts iiad 

 been made to trace a like orderly development in the 

 physical life of plants and animals (Buffon, Erasmus 

 Darwin, etc.), but neither was the proof cogent nor the 

 process intelligible. Cautious people therefore, and 

 those whose prepossessions inclined them to adopt a 

 very different orig-in for terrestrial life, held during all 

 this time a position of some strength against speculative 

 philosophers who tried to explain the variety and perfec- 

 tion of living" nature by unconscious and unintelligent 

 factors. 



About the year 1840 the doctrine of the fixity of 

 species seemed to be victorious. Cuvier's knowledge 

 and skilful advocacy had a few years before over- 

 powered Geoffroy St. Hilaire's conception of a common 

 plan of structure pervading the whole animal kingdom, 

 and the new Philosophie Anatomique was laid on the 

 shelf, side by side with the Philosophie Zoologique of 



