300 Ward. — On Relations between Host and Parasite 
nearly the same — these species being so closely allied as to 
be regarded by some authorities as mere varieties. 
But matters are different when it comes to attacking 
B. maximus, B. sterilis , dec. We may suppose the food and 
the resistances offered to the mycelium by these species to 
differ considerably from those which existed in the tissues of 
B. mollis , though not perhaps sufficiently so to present a barrier 
incapable of being overcome by an odd spore varying from the 
normal here and there. 
I regard this matter as exactly similar in kind (however 
different in degree it may be) to that where a carbo-hydrate- 
loving Fungus is sown on a proteid, or when a Yeast adapted 
to one kind of sugar is placed in another kind,, or where we 
attempt to rear dung-fungi on gelatine, wood-fungi on potato, 
and so forth. Numerous other illustrations can be given, but, 
not to multiply analogies unnecessarily, I will merely refer 
to the well-known case of Bacteria, distinct races of which can 
be produced by slight differences of nutrition and training. 
Such cases also remind us that it is possible to educate 
a given Fungus or Bacterium to feed on very different media 
by gradually accustoming it to the changed conditions ; and 
I suppose that when the Uredospores from B. mollis are sown 
on B. secalinus and B. racemosus as well as B. mollis , and find 
that it does best on the latter, next best on B. secalinus , and 
worst on B. racemostis, that such ‘ education ’ is substantially 
what is occurring, because if I now take the Uredospores 
developed in consequence of this infection on B. secalinus and 
sow them on the same three species, it is B. secalinus which 
proves the best host for them. 
It is indeed very probable that by gradual variation and 
adaptation the Fungus can pass to all or nearly all the species 
of Bromes in turn, even to such as have hitherto appeared 
immune. 
That by gradual passage from variety to variety, and from 
one species to a closely allied one, such spores may be 
gradually adapted to different hosts in nature, seems an obvious 
corollary from the facts. 
