THE GEO(rRAl'lIICAL DISTRIBUTION OF PLANTS. 107 



or controlling agencies ; they are not the distributing agencies. 

 And the}' are the controlling agencies for this reason : every species 

 of plant, withont exception, has certain definite maximum and 

 minimum points both of temperature and moisture within the range 

 of which it can live, outside of which it must perish ; and moreover 

 each has within those limits a certain optimum point of temperature 

 and optimum quantity of moisture at which it flourishes best, and 

 each plant, other things being equal, could flourish anywhere on the 

 earth's surface where these best conditions are realized, were the 

 field open for it and couhl it hut get there. But that is the great if ; 

 plants have comparatively very small powers of travelling them- 

 selves, and geological history, with its forced migrations, has driven 

 them into the regions where we find them and kept them out of 

 others to which they are equally well fitted. You can understand 

 better now what I meant at the beginning when I said that a plant 

 is the creation of its history and its surroundings, and that these 

 have operated to make it just what it is and to place it just where 

 it is. And I trust you will understand it still better before I shall 

 have finished this lecture. 



These being the limiting agencies, we are now prepared to look 

 at the distributing agencies and to note what effects have been pro- 

 duced on vegetation by their combined action. These distributing 

 agencies are three : — 



I. Natural methods of dissemination. 

 TI. Influence of man, direct and indirect. 



III. Geological changes — the changes which have in past times 

 compelled groups of plants to migrate from place to place. 



The first and second of these deal chiefly with single species, 

 and are therefore of less significance. The third has to do with 

 great masses of species historically connected, or Floras. 



The subject of the natural dissemination or scattering of seeds is 

 one of the most interesting topics in botany, and I am sure a pre- 

 sentation of it would prove of great interest to you. 



Plants have developed the most remarkable devises to secure a 

 wide scattering for their seeds. But their bearing upon our present 

 subject is limited by the fact that these devices are rarely adapted 

 to carry the species beyond a limited distance, and very rarely 

 indeed are they of a character to enable the plant to surmount the 

 natural barriers imposed by mountain ranges or wide seas. Of the 

 exceptions one of the most important is found in the coconut. 

 This fruit has a thick but very light coating, which is unusually 



