54 FORREST SHREVE 



of habitats, differing by greater or less degrees. The major task 

 of plant geography is to determine the causal relation of these 

 environic conditions and sets of conditions to the manifold facts 

 of distribution. Each successive geological epoch has presented 

 problems of the same kind that are presentd by the distribution 

 of plants today. We can scarcely hope to learn more than the 

 merest outlines regarding the ancient distributions of plant 

 stocks, and the history of the genera which are still represented 

 in our floras. We can, however, learn much in regard to, the 

 influences at work at the present time in determining the general 

 and the habital distribution of plants. 



It is our custom to speak of the ''historical factors" and the 

 present factors operative in determining plant distribution. We 

 must bear in mind that when we speak of historical factors we 

 are embracing in the term two very dissimilar things which h^ve 

 registered a combined effect. We must look to evolutionary 

 history and to paleobotany to tell us where, for example, the 

 genus Sequoia originated, at what epoch, and from what stock. 

 W^e must look to paleobotany and paleoclimatology to tell us 

 what have been the movements, the extensions and retreats of 

 Sequoia. The initiation of a species is — from our standpoint — 

 purely an evolutionary event; the history and fate of a species 

 after its initiation are decided by the changes in orography or 

 climate which it encounters, with the possible participation of 

 physiological changes in the stock of such nature as to be un- 

 accompanied by morphological modifications of diagnostic value. 

 It is conducive to clearer thinking, therefore, to distinguish 

 between the evolutionary factors and the paleoclimatic factors 

 which compose the historical factor. This distinction has its 

 principal value in compelling us to regard the present distribu- 

 tional phenomena of the earth as merely a moment in the pro- 

 longed and incessantly active procession of change due to secular 

 or sudden changes of climate, or to destructive or constructive 

 events in surface geology, and discontinuously marked by evo- 

 lutionary activity. We are able to realise that there is no gulf 

 between the climate of the past and that of the present; and that 

 there has been no sudden, extensive or unaccountable read- 

 justment of distributions. 



