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.1— 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 ¢ntroduced 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 (Ipomea pur- 
pureda) 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, seiected by C. 8. Sargent, Vol. II, pp. 234-242. 
2Rather more than three thousand two hundred by actual count and 
estimation. 
