Kerner designates a willow, _Salix ehrhartiana_ as a constant hybrid
between _Salix alba_ and _S. pentandra_. _Rhododendron intermedium_ is
an intermediate form between the hairy and the rusty species from the
Swiss Alps, _R. hirsutum_ and _R. ferrugineum_, the former growing on
chalky, and the other on silicious soils. Wherever both these types of
soil occur in the same valley and these two species approach one
another, the hybrid _R. intermedium_ is produced, and is often seen to
be propagating itself abundantly. As is indicated by the name, it
combines the essential characters of both parents.
_Linaria italica_ is a hybrid toad-flax between _L. genistifolia_ and
_L. vulgaris_, a cross which I have repeated in my garden. _Drosera
obovata_ [268] is a hybrid sundew between _D. anglica_ and _D.
rotundifolia_. _Primula variabilis_ is a hybrid between the two common
primroses, _P. officinalis_ and _P. grandiflora_. The willow-herb
(_Epilobium_), the self-heal (_Brunella_) and the yellow pond-lilies
(Nuphar) afford other instances of constant wild hybrids.
Macfarlane has discovered a natural hybrid between two species of sundew
in the swamps near Atco, N.J. The parents, _D. intermedia_ and _D.
filiformis_, were growing abundantly all around, but of the hybrid only
a group of eleven plants was found.
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