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 cultivation. They emerge from their obscure natural retreats at the approach 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. artemisiæfolia (Roman wormwood) is one of the farmer's most persistent pests, and resists all efforts at extermination. The cocklebur and thorny clotbur (Xanthium strumarium and X. spinosum) warn us of their disagreeable presence wherever we go. Polygonum 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 angustifolium (great willow-herb), Gnaphalium polycephalum (common everlasting), 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 observation to Europe or Asia to render all that has been said of indigenous 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 species has had a native habitat somewhere, and its preference for proximity to human habitations is unexplainable on any theory of original adaptation.
Illustrations 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-
