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quite in accordance with the fact iw his suggestion that it was after 
the frost left the roots that the injury began. If Prof. Shaler will 
remember that there is an enormous evaporation going on from 
plants exposed to a dry atmosphere, and that this takes place 
whether there be frozen soil about the roots or not, he will I think 
understand how a plant may become exhausted of itself, without 
waiting for the thaw. If there be a very dry atmosphere, and the 
roots nearly all encased in frost at the same time, it is still more 
difficult to supply this waste. The deeper the frost the greater 
the difficulty, and the more evaporating surface, as in evergreens, 
the greater the risk. 
The destruction by drought and not by the absolute degree of 
frost being conceded, there remains nothing but to apply the law 
to general science as Prof. Shaler suggests; a dry atmosphere 
becomes a destructive agent as well as frost, and those plants 
which part with their moisture the most readily, as a climate 
passes from moist to dry, must be the first to disappear. In my 
grounds I had large quantities of American hornbeam side by side 
with the English species. These last were all killed to the 
ground,— the others uninjured. This shows that the American 
Species can resist evaporation better than the European. It is 
difficult to decide from an evolutionary point of view which of 
these two very closely allied species had the priority of origin. If 
we accept the proposition that in water was the beginning of plant 
life, we might infer that development has been in the direction of 
the dry atmosphere, and thus arrive at the conclusion that by 
natural selection the American is an offshoot from the European. 
In my grounds also the Liriodendron suffered terribly. I had ten 
thousand from one to five feet high killed to the ground, but all 
above this were uninjured, as their roots were deep in the ground, 
and could supply the waste of sap without much destruction from 
the frost. But the fact of the younger ones drying up so easily, 
Shows that this tree was not created for a dry winter climate. 
We must infer that they are either immigrants, or that the climate 
` has changed since their first appearance. And then again arises 
another suggestion. Suppose the future seasons should regularly 
repeat the last, would “natural selection” be sufficient to produce 
_ Some less liable to loss by evaporation, as we have supposed may 
have been the case with the hornbeam? Would this change to a 
greater winter aridity, if continuous, give rise to a new species of 
Liriodendron ? 
