t in the Botanic Gardens at Calcutta the
great heat causes apple-trees to become fastigate; and we {362} thus
see the same result following from the effects of climate and from an
innate spontaneous tendency.[764]

In foliage we have variegated leaves which are often inherited; dark
purple or red leaves, as in the hazel, barberry, and beech, the colour
in these two latter trees being sometimes strongly and sometimes weakly
inherited;[765] deeply-cut leaves; and leaves covered with prickles, as
in the variety of the holly well called _ferox_, which is said to
reproduce itself by seed.[766] In fact, nearly all the peculiar
varieties evince a tendency, more or less strongly marked, to reproduce
themselves by seed.[767] This is to a certain extent the case,
according to Bose,[768] with three varieties of the elm, namely, the
broad-leafed, lime-leafed, and twisted elm, in which latter the fibres
of the wood are twisted. Even with the heterophyllous hornbeam
(_Carpinus betulus_), which bears on each twig leaves of two shapes,
"several plants raised from seed all retained the same
peculiarity."[769] I will add only one other remarkable case of
variation in foliage, namely, the occurrence of two sub-varieties of
the ash with simple instead of pinnated leaves, and which generally
transmit their character by seed.[770] The occurrence, in trees
belonging to widely different orders, of weeping and fastigate
varieties, and of trees bearing deeply cut, variegated, and purple
leaves, shows that these deviations of structure must result from some
very general physiological laws.

Differences in general appearance and foliage, not more strongly marked
than those above indicated, have led good observers to rank as distinct
species certain forms which are now known to be mere varieties. Thus a
plane-tree long cultivated in England was considered by almost every
one as a North American species; but is now ascertained by old records,
as I am informed by Dr. Hooker, to be a variety. So again the _Thuja
pendula_ or _filiformis_ was ranked by such good observers as Lambert,
Wallich, and others as a true species; but it is now known that the
original plants, five in number, suddenly appeared in a bed of
seedlings, raised at Mr. Loddige's nursery, from _T. orientalis_; and
Dr. Hooker has adduced excellent ev