268 MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 



of pond-weeds, water-crowfoots, hornwort, and various plants of the kind. The 

 second generation is produced in greater abundance than the first, and the third 

 develops more luxuriantly than the second. The third may be followed by a fourth, 

 fifth, and sixth. Each successive generation crushes out and supplants the one 

 preceding it. 



As on the rocky heights and in the roaring torrents of mountains, so also on the 

 sandy plain and in the depths of the sea, a perpetual variation in the nature of the 

 vegetation is taking place. At all times and in all places we see younger genera- 

 tions displacing the older and building upon the foundations laid by their pre- 

 decessors. The first settlers have a hard fight with uncompromising elements to 

 seize possession of the lifeless ground. Years go by before a second generation is 

 enabled to develop in greater luxuriance upon the earth prepared by the first 

 occupiers; but there is no cessation in the productive and regulative effects of 

 vegetable life, and its energy and aptitude in the work result in the erection of its 

 green edifices over wider and wider areas. New germs are established upon the 

 mouldered dust of dead races, and others on the plant forms adapted to the altered 

 substratum, and so, for hundreds and thousands of years, the changes go on, until 

 at length the tops of forest-trees wave above a black and deep soil, the battle-field 

 of a number of bygone generations. Thus, the life of plants, like that of the human 

 race, has its epochs and its history: as in the one so in the other a continual 

 struggle prevails; processes of ousting and of renovation are always in progress, and 

 there are ever new arrivals upon and departures from the scene. 



