THE (JEOGHAPHKAL DISTRIBUTIOX OF PLANTS. 103 



and tliis focus or resultant is to the forces ver}' much wliat its 

 centre of gnivity is to a complexly irregular liod}'. It represents a 

 most delicately adjusted lialance of conditions — a state of uiistal)le 

 equilibrium, which may be altered by the slightest change in any one 

 of the forces ; and the jilant must become something different just 

 in proportion to the intensity of the change. These influences are 

 none the less real because so minute, and their study is the study 

 of plant life. To complete our conception of this kind of study, 

 it must be added that the scientific botanist believes, in his working 

 hours at least, in the uniform immutability of those series of con- 

 catenations of events which we call the laws of Nature ; and more- 

 over, leaving the unknowable for the use of the metaphysician, he 

 acts upon the belief that all things in nature are knowable, can 

 we but sufficiently refine our methods of investigation. 



Now the point of these observations lies in the application thereof 

 to our present subject. Each and every plant has its place on the 

 earth's face fixed by a tremendously complex set of influences, 

 some strong, some weak ; some acting through heredity, some from 

 environment ; some from this, some from that ; the end and result 

 of all of them being to make the plant just what it is, and to place 

 it just where it is ; and any change in any of these influences will 

 disturb the balance, move the focus, and cause a corresponding 

 change in the plant, which will vary directly as the influence. And 

 in this brief survey of the field we can but consider the broader 

 pencils or groups of forces determining the geographical distribu- 

 tion of plants, taking time to resolve none of them into lesser 

 groups, much less into smaller details. 



The great controlling or limiting phj^sical agencies in distribu- 

 tion then are these : 



I. Heat and moisture, or in other words, climate. 



II. The past geological history of places. 



And this is true not only of great areas, but as well of the most lim- 

 ited, and there is not a square mile of land in New England which 

 will not furnish illustrations of this principle. The main cause which 

 confines the Cactus to the desert and the great Aroids to the damp 

 forests, is the same, but in lesser degree, which places on our dry- 

 est knolls our Saxifrage and P^verlasting, and in our marshy pools 

 our Iris and Calla. It is but the same cause in different degree 

 which allows the low Arctic herbs to exist upon the Himalaj^a, 

 and the white Potentilla upon our own most exposed and coldest 



