CHAPTEE XXI. 



The Struggle for Existence and the Survival of the 

 Fittest. 1 



238. Weeds. — Any flowering plant which is troublesome 

 to the farmer or gardener is commonly known as a weed. 

 Though such plants are so annoying, from their tendency to 

 crowd out others useful to man, they are of extreme interest 

 to the botanist on account of this very hardiness. The prin- 

 cipal characteristics of tlie most successful weeds are their 

 ability to live in a variety of soils and exposures, their rapid 

 growth, resistance to frost, drought, and dust, their unfitness 

 for the food of most of the larger animals, in many cases 

 their capacity to accomplish self-fertilization, in default of 

 cross-fertilization, and their ability to produce many seeds 

 and to secure their wide dispersal. Xot every weed com- 

 bines all of these characteristics. For instance, the velvet- 

 leaf or butter-print ^ common in corn-fields, is very easily 

 destroyed by frost; the pigweed and purslane are greedily 

 eaten by pigs and the ragweed by some horses. The horse- 

 radish does not usually produce any seeds. 



It is a curious fact that many plants which have finally 

 proved to be noxious weeds have been purposely introduced 

 into the country. The fuller's teasel, melilot, horseradish, 

 wild carrot, wild parsnip, tansy, ox-eye 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. 



1 See Darwin's Origin of Species, Cliapters III and rv. 



2 Atmtiltm Aviceniise. 



