58 ORIGIN OF CULTIVATED PLANTS. 



to the South Sea Islands at an early epoch, or from Asia 

 and from Australia to America at a time sufficiently 

 remote for its cultivation to have been early diffused 

 from the Southern States to Brazil and Chili. We must 

 assume a prehistoric communication between Asia and 

 America, or adopt another hypothesis, which is not in- 

 applicable to the present case. The order Convolvulacece is 

 one of those rare families of dicotyledons in which certain 

 species have a widely extended area, extending even to 

 distant continents.^ A species Avhich can at the present 

 day endure the different climates of Virginia and Japan 

 may well have existed further north before the epoch of 

 the great extension of glaciers in our hemisphere, and 

 prehistoric men may have transported it southward 

 when the climatic conditions altered. According to 

 this hypothesis, cultivation alone preserved the species, 

 unless it is at last discovered in some spot in its ancient 

 habitation — in Mexico or Columbia, for instance.^ 



Beetroot — Beta vulgaris and B. onaritinna, Linnaeus ; 

 beta vulgaris, Moquin. 



This plant is cultivated sometimes for its fleshy root 

 (red beet), sometimes for its leaves, which are used as a 

 vegetable (white beet), but botanists are generally agreed 

 in not dividing the species. It is known from other 

 examples that plants slender rooted by nature easily 

 become fleshy rooted from the effects of soil or cultivation. 



The slender-rooted variety grows wild in sandy soil, 

 and especially near the sea in the Canary Isles, and all 

 along the coasts of the Mediterranean Sea, and as far as 

 the Caspian Sea, Persia, and Babylon,^ perhaps even as 



^ A. de Canclolle, Geogr. Bot. Raisonne, pp. 1041-1043, and pp. 

 516-518. 



2 Dr. Bretschneider, after having read the above, wrote to me from 

 Pekin that the cultivated sweet potato is of origin foreign to China, 

 according to Chinese authors. The handbook of agriculture of Nung- 

 chang-tsuan-shu, whose author died in 1633, asserts this fact. He 

 speaks of a sweet potato wild in China, called chu, the cultivated species 

 being Jcan-chu. The Min-shu, published in the sixteenth century, says 

 that the introduction took place between 1573 and 1620. The American 

 origin thus receives a further proof. 



2 Moquin-Tandon, in Prodromus, vol. xiii. pt. 2, p. 55 j Boissier^ 

 Flora Orientalis, iv. p. 898 ; Ledebour, Fl. Rossica, iii. p. 692. 



