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vegetation, but consisting of fewer species. So too with respect 

 to climate, some plants like a moderate climate, others a more 

 extreme one. In the west of England, the summers are for the 

 most part cooler, and the winters milder, than in the eastern 

 counties, and accordingly, though many species of plants may 

 grow indifferently on both sides of the kingdom, each side has 

 certain species not found in the other. Some plants again thrive 

 best in a humid atmosphere, while others are able to maintain 

 themselves on the most barren spots — even on the tops of walls. 

 Some species are altogether aquatic, growing in rivers, or pools, 

 or marshes. Many species again grow only by the sea, or where 

 they can have the influence of sea breezes or brackish water. 



Such then being the various conditions required by plants for 

 their proper growth and development, let us suppose a particular 

 species to have gained a footing in some country, the requisite 

 conditions of which are favourable to its diffusion. If it had 

 that country entirely to itself it would quickly overrun it. This, 

 however, is never likely to be the case. Other species are at 

 liberty to enter in with it, as when our own islands were first 

 stocked -ndth plants, according to the theory above alluded to ; 

 it then has to contend with these others, all striving together for 

 the possession of the soil ; and any advantages the former 

 possessed, had it been alone, may be counteracted by greater 

 advantages possessed by the latter. The climatal and other 

 couditions of the country may be yet more favourable for its 

 companions than for itself. Or the former may be plants of 

 larger size and quicker growth, or of more robust habit, soon 

 overpowering the smaller and weaker species. Or they may have 

 fewer enemies to contend mth wliilst in a seedling state. Mr. 

 Darwin, " on a piece of ground three feet long and two wide, 

 dug and cleared, and where there could be no choking from other 

 plants, marked all the seedlings of our native weeds as they 

 came up, and out of the 357 no less than 295 were destroyed, 

 chiefly by slugs and insects." Thus we see there is constantly 



