HYBRIDISATION VIEWED FROM SYSTEMATIC BOTANY. 201 
Salvia sylvestris, a hybrid from S. nemorosa and pratensis, abounds 
in dry meadows all over the low country south of Vienna. It is an un- 
dulating country, with elevations of boulders and clay. On the rising 
ground and gentle slopes, S. nemorosa is the prominent feature, but in 
hollows of black earth and humus S. pratensis luxuriates. The two 
kinds of habitat pass into each other gradually, those common to both 
consisting of dry meadowland in which the former parent does not 
thrive, and is seldom seen, while it is too dry for pratensis, which is also 
poorly represented. But the hybrid, being intermediate in character, 
thrives there exceedingly ; is much visited by insects; and ripens seeds 
as well as either parent, of which experiment has shown that 60 per cent. 
are fertile. Thus the hybrid, which is scattered all over this meadow- 
land, manifests all the characteristics essential to our conception of a 
species (p. 588). 
Nuphar intermedium, a hybrid from N. lutewn and pumilum, occurs 
in lakes in the Black Forest and in the Vosges, in scattered localities of 
North Germany, Central and North Russia and Sweden, and as far north 
as Lapland, in the north being more abundant than either parent—in fact 
it passes their northern limit, and occurs without them. Here it multiplies 
from seed without change of form, and in fact has established itself, to 
all appearances, as a species; yet its origin has been proved by artificial 
crossing. The reason why the hybrid has been able to extend its range 
further north is that it matures seeds in rather shorter time than either 
parent, and thus we may say that the northern limit of each is deter- 
mined by its ability to ripen its fruit. N. /uwtewm flowers last, and is the 
last to ripen its seeds; consequently as we proceed northward we find 
that it is the first to fail. NN. pumilum ripens a little earlier, and conse- 
quently gets a little further north, while V. intermediwm is able to extend 
itself beyond either. As, however, hybrids can only occur where the 
parents grow together, it is clear that the extension of N. intermediwm 
northwards beyond the limits of N. lwtewm has been by its own seeds, 
and there at all events it is autonomous (p. 589). 
Sometimes a hybrid is fonnd in company with one parent only, or 
with one in one locality and both in another ; sometimes even where both 
are absent; and this may either be due to diffusion from the original 
birthplace, or, in some cases, to displacement of the parents by changed 
conditions. In fact, a hybrid which is fertile with its own pollen may 
easily extend its range if conditions are favourable, and thus originate a 
hybrid race, which, as we have seen, may present all the essential charac- 
teristics of a species. Ido not, however, propose to call them “ species.”’ 
They have certainly not arisen by divergence of character, but rather by 
the sudden fusion of essentially distinct branches, forming points from 
which new branches may themselves arise. Here, then, is a new factor in 
the branching of our genealogical tree, or, at all events, one which has 
as yet only been very imperfectly realised, and it emphasises the import- 
ance of the subject under discussion to all systematists who wish to go 
beyond the mere identification of their plants. 
To what extent, then, does hybridisation occur in nature? Kerner 
estimates that something like a thousand natural hybrids have been 
found in Europe during the last forty years, and there is no reason to 
