Ward . — Recent Researches on the Parasitism of Fungi . 43 
also to those from B. mollis and B. patulus ( Serrafalcus ), and, in the case 
of its variety villosus at any rate, also to spores from B . sterilis ( Steno - 
bromus) as well. 
So that B. Arduennensis is a ‘ bridging species’ by means of which 
P. dispersa can pass into three of the five groups of Bromes. 
I therefore concluded ( 191 , p. 150): — ‘It seems to me that we have 
in these cases of “ bridging species ” the clue to an explanation of a phe- 
nomenon which must be assumed to occur in nature, whatever hypothesis 
we accept regarding the origin and signification of adaptive parasitism, 
viz. the passage of the Fungus from species of one circle of alliance to those 
of another , in spite of the fact that it is usually closely adapted to species 
of one section of the genus only. ... We may suppose a uredospore from 
B. sterilis to infect B. Arduennensis var. villosus , and the crop of spores 
produced on this to further infect B. Arduennensis : thence the Fungus 
could pass to B. secalinus , and, further, to B. brizaeformis. According to 
Table III it would appear possible that an odd spore from the latter 
could infect B. carinatus y and if so, this would have for result the passage of 
the Fungus to four out of the five sections of the genus.’ 
Further experience only confirms the above, and convinces me that in 
these ‘ bridging ’ forms we have the clue to the phenomenon of the ever- 
widening cycle of adaptation. 
In nine hundred and ninety-nine times out of the thousand the spores 
educated by, or adapted to, a given small circle of host-plants cannot 
successfully break through the defences of another circle ; but in the 
thousandth case a single spore just manages to infect the alien host, and, 
once established, its progeny can go on infecting the new host. Or, it may 
be, that in nine hundred and ninety-nine times out of the thousand the 
spores from all kinds of alien sources fall on a given host, and it successfully 
resists any infection even though the germ-tubes enter the stomata ; but 
the thousandth individual, weak in resisting power — whether machinery, or 
substance, or physiological activity — lets the enemy in, and it is then 
established. 
The evidence compels us to believe that the host reacts upon and 
affects the physiological powers of the Fungus : these effects are invisible, 
and have no distinguishable morphological impress on the spores. 
But is this always the case? De Bary’s results with Aecidium abieti- 
num (12) point the other way : they suggest that very slight morphological 
results follow according as the Fungus has passed its alternate phase 
( Chrysomyxa ) on Rhododendron or on Ledum , and many similar cases can 
be quoted. 
If this is once established, then we have the clue to the graduations 
of morphological differences sufficiently distinct for the determination 
of species. 
