18 coisrcLusioisrs on an'cieitt climates. 



Even if we suppose an identity of species, of race, and of habit, 

 to be established between a given ancient and modem plant, the 

 negative fact that the latter will not grow now where it flourished 

 two thousand years ago, does not, in all cases, prove a change of 

 climate. The same result might follow from the exhaustion of 

 the soil,* or from a change in the quantity of moisture it habitu- 



ing the laws of acclimation of plants as maize or Indian com. Maize is grown 

 from the tropics to at least lat. 47^ in Northeastern America, and farther 

 north in Europe. Every two or three degrees of latitude bring you to a new 

 variety, with new climatic adaptations, and the capacity of the plant to ac- 

 commodate itself to new conditions of temperature and season seems almost 

 unlimited. 



Many persons now living remember that, when the common tomato was 

 first introduced into Northern New England, it often failed to ripen ; but, in 

 the course of a very few years, it completely adapted itself to the climate, and 

 now not only matures both its fruit and its seeds with as much certainty as any 

 cultivated vegetable, but regularly propagates itself by self-sown seed. Me- 

 teorological observations, however, do not show any amelioration of the sum- 

 mer climate in those States witMn that period. 



It may be said that these cases — and indeed all cases of a supposed acclima- 

 tion consisting in physiological changes — are instances of the origination of 

 new varieties by natural selection, the hardier maize, tomato, and other veg- 

 etables of the North, being the progeny of seeds of individuals endowed, ex- 

 ceptionally, with greater power of resisting cold than belongs in general to the 

 species which produced them. But, so far as the evidence of change of cli- 

 mate, drawn from a difference in vegetable growth, is concerned, it is imma- 

 terial whether we adopt this view or maintain the older and more familiar 

 doctrine of a local modification of character in the plants in question. 



Maize and the tomato, if not new to human use, have not been long known 

 to civilization, and were, very probably, reclaimed and domesticated at a much 

 more recent period than the plants which form the great staples of agricul- 

 tural husbandry in Europe and Asia. Is the great power of accommodation 

 to climate possessed by them due to this circumstance ? There is some reason 

 to suppose that the character of maize has been sensibly changed by cultiva- 

 tion in South America ; for, according to Tschudi, the ears of this grain 

 found in old Peruvian tombs belong to varieties not now known in Peru.— 

 Travels in Peru, chap. vii. See important observations in ScHiiBELER, Die 

 Pflanzenwelt Norwegens {Allgemeiner Theil), Christiana, 1873, 77 and follow- 

 ing pp. 



* The cultivation of madder is said to have been introduced into Europe by 

 an Oriental in the year 1765, and it was first planted in the neighborhood of 

 Avignon. Of course, it has been grown in that district for less than a cen- 

 tury ; but upon soils where it has been a frequent crop, it is already losing 

 much of its coloring properties. — Lavergne, ^conomie Rurale de la France, 

 pp. 259-291. 



I believe there is no doubt that the cultivation of madder in the vicinity ot 



