RELATIONSHIPS OF METASPERMAE 587 
The general factors in floral 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 natural 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 wellas 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 allan 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- 
