THE STRUGGLE FOR EXISTENCE 201 



wild carrot, wild parsnip, tansy, oxeye daisy, and field-garlic 

 are only a few of the many examples of very troublesome 

 weeds which were at first planted for use or for ornament. 



246. Origin of Weeds.i — By far the larger proportion 

 of our weeds are not native to this country. Some have 

 been brought from South America and from Asia, but 

 most of the introduced kinds come from Europe. The 

 importation of various kinds of grain and of garden-seeds, 

 mixed with seeds of European weeds, will account for the 

 presence of many of the latter among us. Others have 

 been brought over in the ballast of vessels. Once landed, 

 European weeds have succeeded in establishing themselves 

 in so many cases, because they were superior in vitality 

 and in their power of reproduction to our native plants. 

 This may not improbably be due to the fact that the Euro- 

 pean and western Asiatic vegetation, much of it consisting 

 from very early times of plants growing in comparatively 

 treeless plains, has for ages been habituated to flourish in 

 cultivated ground and to contend with the crops which 

 are tilled there. 



247. Plant Life maintained under Difficulties Plants 



usually have to encounter many obstacles even to their 

 bare existence. For every plant which succeeds in reach- 

 ing maturity and producing a crop of spores or of seeds 

 there are hundreds or thousands of failures, as it is easy 

 to show by calculation. The morning-glory {Ipomoea pur- 

 purea) is only a moderately prolific plant, producing, in an 

 ordinary soil, somewhat more than three thousand seeds.^ 

 If all these seeds were planted and grew, there would 



1 See the article Pertinacity and Predominance of Weeds, in Scientific 

 Papers of Asa Gray, selected by C. S. Sargent, Vol. 11, pp. 2.^-242. 



2 Rather more than three thousand two hundred by actual count and 

 estimation. 



