RELATIONSHIPS OF METASPERMAE 587 



The general factors in fioral differences. These are, as 

 indicated, three in number. In order of their importance they 

 are history, humidity, distance. The third is geographical, the 

 second geographical and cosmical, the first biological. The 

 first is the most complex, upon analysis; the third is least 

 complex. It is apparent, then, that if the explanation of such 

 a series of phenomena as is presented by the plant-population 

 of a natu ral district like the valley of the Minnesota is to be 

 attempted, it must be through a knowledge of geographical, 

 climatological and biological conditions. Not only present 

 conditions but past conditions must be comprehended in such 

 an explanation. The knowledge of past geography, past 

 climatology and past biologic phenomena is as essential as the 

 knowledge of these factors as they exist today. Geographical 

 distribution of plants is therefore based upon geology as well 

 well as upon topography, upon development as well as upon 

 classification, upon embryology as well as upon anatomy. It 

 is a study in evolution no less than in systematics. Thus the 

 difficulty of the problems pressing for solution is seen to be 

 greater as they come to be comprehended. The position of 

 an individual plant in one locality rather than in another be- 

 comes a matter for historic study, and such is the interde- 

 pendence of all portions of the universe that the final explana- 

 tion of what is apparently a single and simple phenomenon is 

 after all an explanation of phenomena in the highest degree 

 multiple and complex. In the scientific, as in the poetic sense, 

 a knowledge of the violet is, at the same time, a knowledge of 

 everything else. 



In the present stages of our knowledge it is apparent that 

 final explanations are remote and that inquiry must pause 

 before its limitations. Partial answers are all that may be 

 offered by partial information. 



In naming the three factors of floral differences it will be ob- 

 served that no classification of the methods by which these 

 differences arose is attempted. Indeed examination a little 

 more intimately will show that the three factors may be re- 

 solved into terms of the first. Distance and humidity, in their 

 relations to the plant-population of the globe, become biological 

 in their significance, and the distances and climate of to-day, 

 considered quite apart from vegetation, are themselves phe- 

 nomena of evolution. The geological history of the earth has 

 had much to do with determining its topography, geography 

 and climate. Therefore the problems of plant distribution be- 



