CONCLUSIONS ON ANCIENT CLIMATES. 19 
Even if we suppose an identity of species, of race, and of 
habit to be established between a given ancient and modern 
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 habitually contains. After a district of country has 
north in Hurope. Every two or three degrees of latitude brings you to a new 
variety, with new climatic adaptations, and the capacity of the plant to 
accommodate 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 avery 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. 
Meteorological observations, however, do not show any amelioration of the 
summer climate in those States within 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 
vegetables of the North, being the progeny of seeds of individuals endowed, 
exceptionally, 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 
climate, from a difference in vegetable growth, is concerned, it is immaterial 
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 
agricultural husbandry in Europe and Asia. Is the great power of accommoda- 
tion 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 
cultivation 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 SCHUBELER, Die 
Pflanzenwat Norwegens (Aligemeiner Theil), Christiania, 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 
century ;.but upon soils where it has been a frequent crop, it is already losing 
