light causes variability rests.

I may here just allude to the appearance of new and valuable varieties
of fruit-trees and of wheat in woods and waste places, which at first
sight seems a most anomalous circumstance. In France a considerable
number of the best pears have been discovered in woods; and this has
occurred so frequently, that Poiteau asserts that "improved varieties
of our cultivated fruits rarely originate with nurserymen."[619] In
England, on the other hand, no instance of a good pear having been
found wild has been recorded; and Mr. Rivers informs me that he knows
of only one instance with apples, namely, the Bess Poole, which was
discovered in a wood in Nottinghamshire. This difference between the
two countries may be in part accounted for by the more favourable
climate of France, but chiefly from the great number of seedlings which
spring up there in the woods. I infer that this is the case from a
remark made by a French gardener,[620] who regards it as a national
calamity that such a number of pear-trees are periodically cut down for
firewood, before they have borne fruit. The new varieties which thus
spring up in the woods, though they cannot have received any excess of
nutriment, will have been exposed to abruptly changed conditions, but
whether this is the cause of their production is very doubtful. These
varieties, however, are probably all descended[621] from old cultivated
kinds growing in adjoining orchards,--a circumstance which will account
for their variability; and out of a vast number of varying trees there
will always be a good chance of the appearance of a valuable kind. In
North America, where fruit-trees frequently spring up in waste places,
the Washington pear was found in a hedge, and the Emperor peach in a
wood.[622]

With respect to wheat, some writers have spoken[623] as if it were an
ordinary event for new varieties to be found in waste places; the
Fenton wheat was certainly discovered growing on a pile of basaltic
detritus in a quarry, but in such a situation the plant would probably
receive a sufficient amount {261} of nutriment. The Chidham wheat was
raised from an ear found _on_ a hedge; and Hunter's wheat was
discovered _by_ the roadside in Scotland, but it is not said that this
latter variety grew where it was found.[624]

Whether our domes