14 THE WORLD OF LIFE 



where alone they are usually to be found. These peculiarities 

 are most strongly marked in the case of plants, and in a less 

 degree among insects and land-shells; and in the former group 

 they are easily seen to depend mainly on such obvious peculiari- 

 ties as soil and moisture, exposure to sun or wind, the pres- 

 ence or absence of woods, streams, or mountains. 



But besides these inorganic causes — soil, climate, aspect, 

 etc. — which seem primarily to determine the distribution of 

 plants, and, through them, of many animals, there are other 

 and often more powerful causes in the organic environment 

 which acts in a variety of ways. Thus, it has been noticed 

 that over fields or heaths where cattle and horses have free 

 access seedling trees and shrubs are so constantly eaten down 

 that none ever grow to maturity, even although there may be 

 plenty of trees and woods around. But if a portion of this 

 very same land is enclosed and all herbivorous quadrupeds 

 excluded, it very quickly becomes covered with a dense vege- 

 tation of trees and shrubs. Again, it has been noticed that 

 on turfy banks constantly cropped by sheep a very large variety 

 of dwarf plants are to be found. But if these animals are 

 kept out and the vegetation allowed to grow* freely, many of 

 the dwarfer and more delicate plants disappear owing to the 

 rapid growth of grasses, sedges, or shrubby plants, which, by 

 keeping off the sun and air and exhausting the soil, prevent 

 the former kinds from producing seed, so that in a few years 

 they die out and the vegetation becomes more uniform. 



A modified form of the same general law is seen when any 

 ground is cleared of all vegetation, perhaps cultivated for a 

 year or two, and then left fallow. A large crop of weeds 

 then grows up (the seeds of which, must have beeoi brought 

 by the wind or by birds, or have lain dormant in the ground) ; 

 but in the second and third years these change their propor- 

 tions, some disappear, while a few new ones arrive, and this 

 change goes on till a stable form of vegetation is formed, often 

 very different from that of the surrounding country. Such 

 changes as these have been observed by local botanists on rail- 



