the distinct specific
characters of the species found on different plants, and to prove
that the parasite of one host will not vegetate upon another,
however closely allied. This admission must not, however, be
accepted as universally applicable, and therefore it should not be
assumed, because a certain parasite is found developed on a special
host, that it is distinct, unless distinctive characters, apart from
habitat, can be detected. _AEcidium compositarum_ and _AEcidium
ranunculacearum_, for instance, are found on various composite and
ranunculaceous plants, and as yet no sufficient evidence has been
adduced to prove that the different forms are other than varieties
of one of the two species. On the other hand, it is not improbable
that two species of _AEcidium_ are developed on the common berberry,
as De Bary has indicated that two species of mildew, _Puccinia
graminis_, and _Puccinia straminis_, are found on wheat.

HYPHOMYCETES.--The moulds are much more universal in their habitats,
especially the _Mucedines_. The _Isariacei_ have a predilection for
animal substances, though not exclusively. Some species occur on dead
insects, others on decaying fungi, and the rest on sticks, stems, and
rotten wood. The _Stilbacei_ have also similar habitats, except that
the species of _Illosporium_ seem to be confined to parasitism on
lichens. The black moulds, _Dematiei_, are widely diffused, appearing
on herbaceous stems, twigs, bark, and wood in most cases, but also on
old linen, paper, millboard, dung, rotting fruit, &c., whilst forms of
_Cladosporium_ and _Macrosporium_ are met with on almost every kind of
vegetable substance in which the process of decay has commenced.

_Mucedines_, in some instances, have not been known to appear on
more than one kind of matrix, but in the far greater number of cases
they nourish on different substances. _Aspergillus glaucus_ and
_Penicillium crustaceum_ are examples of these universal _Mucedines_.
It would be far more difficult to mention substances on which
these moulds are never developed than to indicate where they have been
found. With the species of _Peronospora_ it is different, for these
are truly parasitic on living plants, and, as far as already known,
the species are confined to certain special plants, and cannot be
made to vegetate on any other. The species which causes the potato
murrain, although liable to attack the tomato, and other species of
_Solanaceae_, does not exte