ii DISTRIBUTION OF SPECIES 13 



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 peculiarities as soil 

 and moisture, exposure to sun or wind, the presence 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 vegetation 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 been 

 brought by the wind or by birds, or have lain dormant in 

 the ground) ; but in the second and third years these change 

 their proportions, 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 railway banks, of which I 

 have given several examples in my Island Life (p. 513, 

 footnote). All these phenomena, and many others which 



