236 Ward . — On Relations between Host and Parasite 
next question which arises is : How does this fungus pass 
successfully through its summer form — the Uredo-stage — if 
its power of direct infection is thus limited ? We know of 
course, since De Bary’s classical proof, that it passes the 
winter in the teleutospore-stage to infect barberries in the 
spring by means of the sporidia borne on its pro-mycelium, 
and so forth. But, as Barclay showed, there are districts 
where the barberry is either wanting or very rare, and never- 
theless the Uredo-form is never wanting on the cereals. To 
some extent the explanation is afforded by the repeated and 
continual infection and re-infection of grasses which follow 
on one after the other as regards the growing season. But 
Eriksson's researches led him to conclude that there is 
evidence of there being limits to this, and although the Uredo 
may thus persist on the weed-grasses, it by no means follows 
that it does so to such an extent as theory has demanded. 
Another idea which arose out of these results was that 
the different specialized forms — which, though not morpho- 
logically distinct are physiologically different one from another — 
are incipient species ; that the particular specialized race 
adapted to growth on wheat, but not on rye or barley, is in 
course of becoming a species, and may during the lapse 
of time actually become a species of Puccinia , which will 
eventually show morphological differences in addition to the 
physiological ones it already shows. Various names have 
been proposed for these physiologically, but as yet not 
morphologically, different varieties of Puccinia graminis. 
Eriksson termed them ‘special forms' ( formae speciales *), 
Rostrup names them ‘Biological species’ (biologische Arten), 
Schroeter chose the term Species sorores, and Magnus calls 
them ‘adapted races’ ( Gewohnheitsrasseil ). Not much of im- 
portance is perhaps to be derived from the terminology, but 
it is pretty clear that each authority recognizes more or less 
clearly some aspect of one and the same underlying idea, 
viz. that each of these specialized forms has received from 
its host an impress, not perceptible in any visible difference 
1 Eriksson, 1 . c., p. 92. 
