678 THE POPULAR SCIENCE MONTHLY. 



crevices between them, is planted in gardens where it thrives equally 

 well. The same is true of the Cacti, which, taken from the arid plains 

 where their indurated watery stalks and branches store up the water 

 which the climate so long denies them, thrive under cultivation with 

 undiminished vitality. The Agave, or American aloe, furnishes a 

 similar illustration, and every few years a gorgeous century-plant 

 blooms under cultivation, to the infinite delight of its owner. 



Equally striking results take place under the influence of man 

 without his design or selection. There are a great many indigenous 

 plants which are rarely found outside of the influence of human culti- 

 vation. They emerge from their obscure natural retreats at the ap- 

 proach of civilization, spread rapidly over fields and pastures, and 

 often become formidable enemies of the farmer and the gardener. 

 Under the general name of weeds they are proscribed and pursued, 

 and no effort is spared for their extermination. They also invade towns 

 and cities, overrun vacant lots, disfigure parks and plats, and force 

 themselves into pavements and " crannied walls." Ambrosia trifida 

 (the great rag-weed) forms forests in waste grounds and neglected 

 gardens. A. artemisiwfolia (Roman wormwood) is one of the farm- 

 er's most persistent pests, and resists all efforts at extermination. 

 The cocklebur and thorny clotbur {Xanthium strumarium and JT. spi- 

 nosum) warn us of their disagi-eeable presence wherever we go. Poly- 

 gonum aviculare (knot-grass) and other species invade our door-yards 

 and threaten to cross our thresholds. Euphorbia maculata (spotted 

 spurge) spreads its prostrate and symmetrical mats over the dry and 

 gravelly walks. Spergularia rubra (sand spurrey) unfolds its rosy 

 petals to the hottest July sun upon the parching bricks beneath our 

 feet. * Erigeron Canadense (horse-weed), Epilobium angustifoliinu 

 (great willow-herb), Gnaphalium polycephalum (common everlast- 

 ing), and a host of other indigenous weeds, overrun the cultivated 

 fields and commons wherever man has impressed his influence upon 

 primitive Nature. 



This phenomenon, however, becomes still more obtrusive when we 

 turn to introduced species. And, if it be claimed that the transfer from 

 waste places in the Old World to similar waste places in the New is 

 not a change of conditions, we have only to remove our point of ob- 

 servation to Europe or Asia to render all that has been said of indige- 

 nous plants applicable also to adventive ones. For, unless we are 

 willing to go further in admitting the transmutation of species than 

 the founders of that doctrine, we must assume that each of these spe- 

 cies has had a native habitat somewhere, and its preference for prox- 

 imity to human habitations is unexplainable on any theory of original 

 adaptation. 



Illustration's on this point would be quite superfluous, as these 

 plants constitute the bulk of all our weeds, and present themselves at 

 every turn. I might mention the ubiquitous ox-eye daisy (Leucanthe- 



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