THE GEOGRAPHICAL DISTRIBUTION OF LIFE 443 



were all absent. We ourselves were excluded from 

 America until navigation reached a certain stage of de- 

 velopment. It is of course true that the so-called 

 historic factors are also in a broad sense environmental ; 

 the restricting ocean, mountain, or desert is part of the 

 environment. It is, however, the peripheral environ- 

 mental, the outer wall, not the medium in which the 

 organism lives and has its being. 



3. On further examination it appears that, these Themuiti- 

 broad distinctions, while useful, are very crude. We Jj^fsaiid 

 like to point out the "cause" of this or that, forgetting the inter- 

 that life is subject to a vast multitude of "causes." events 

 Tennyson had this in mind when he wrote : 



Flower in the crannied wall, 



I pluck you out of the crannies, 



I hold you here, root and all, in my hand, 



Little flower but if I could understand 



What you are, root and all, and all in all, 



I should know what God and man is. 



It required the combined forces of the universe to pro- 

 duce the flower, and to ask why it was in the crannied 

 wall is ultimately to raise more questions than any man 

 can hope to answer. Nevertheless, we may by search- 

 ing determine many things, and the study of geograph- 

 ical distribution leads us through winding paths to 

 many remarkable conclusions. 



4. To illustrate the methods of biogeography, we may Methods of 

 take any small area of ground on which plants are ge0 graphy 

 growing. We shall suppose that the plants found are 



the following : sunflower, dandelion, prickly-pear cac- 

 tus, thistle, burdock, and snowberry. Determine the 

 species, and then look up the known distribution of the 

 genera and species. We find that they can be classified 

 as follows : 



