8 THE VEGETATION OF THE UNITED STATES. 



is, indeed, to face a problem of considerable complexity. Even in the 

 presence of the results of our own work we are not prepared to main- 

 tain that the distribution of all species is at the present time strictly 

 controlled by the complex of physical conditions for which we have 

 tried to derive numerical values. We are in the position, however, of 

 being convinced that certain species are thus controlled, the evidence 

 in this direction being partly our own and partly due to an analysis 

 of the work of others. 



It is very generally maintained that the distribution of many plants 

 is due to certain "historical factors,^' which is merely to say certain 

 physical conditions which have operated in the past, or the conditions 

 which determined the distribution of the ancestral stock of the plant 

 in question. When we speak of the historical factors and the present 

 factors operative in determining plant distribution, we must bear in 

 mind that we are embracing in the former term two very dissimilar 

 things which have registered a combined effect. We must look to 

 evolutionary history and to paleobotany to tell us where a particular 

 genus originated, at what epoch, and from what stock. We must look 

 to paleobotany and paleoclimatology to tell us what have been the 

 movements, the extensions, and the retreats of this genus. The 

 initiation of a species is, from our standpoint, purely an evolutionary 

 event; the history and fate of the species after its initiation are con- 

 sidered as dependent upon the changes in orography or climate which 

 it may encounter, always with the possible cooperation of physiological 

 changes in the stock which are unaccompanied by morphological 

 modifications of diagnostic value. It is conducive to clearer thinking, 

 therefore, to distinguish between the evolutionary factors and the 

 paleochmatic factors which compose the historical factor. This dis- 

 tinction has its principal value in compelling us to regard the present 

 distributional phenomena of the earth as merely a momentary stage 

 in the prolonged 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 evolutionary 

 activity. We are made to realize 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 readjustment of distributions. 



The role of the evolutionary factor can not be escaped in any con- 

 sideration of distribution. For example, we owe it to facts in the 

 history of the great plant stocks that Yucca arhorescens occurs in the 

 Mohave Desert of Southern California and that Aloe dichotoma, 

 somewhat similar to it in form, grows in the deserts of Southwest 

 Africa. It is likewise a part of phylogenetic history that very many 

 plants were formerly confined to particular regions, whereas they now 

 have been introduced overseas, into climates which prove to be wholly 

 congenial to them. However, the reasons for the distributional ranges 



