32 Ward . — Recent Researches on the Parasitism of Fungi . 
Eriksson’s Criticisms. 
In 1903 Eriksson made reply (66) to my paper in an article entitled 
‘ The Researches of Professor H. Marshall Ward on the Brown Rust of 
the Bromes and the Mycoplasm Hypothesis/ in which he courteously 
appreciates the work, but at the same time maintains his previous position, 
insisting that my cultural and histological results have little or nothing to 
do with the matter. 
Of my pure cultures in test-tubes ( 193 , 1902, p. 451) he says: — ‘The 
pure cultures in test-tubes, described in 1902, where the results were 
negative in the cases when no infective substance was introduced, prove 
no more against the theory in question than do the numerous experiments 
with cultures, equally negative in their results, which I myself had carried 
out in isolated glass houses during the years 1892-8.’ 
This I concede : in both cases the absence of positive results is, so 
far as it goes, dead against the existence of ‘ mycoplasm ’ in the seedlings 
or plants. 
Eriksson then proceeds to state that he ventures to regard the existence 
of mycoplasm ‘ as proved, at least until sufficiently comprehensive proofs 
to the contrary have been produced from some other quarter.’ Surely this 
is a travesty of the kind of logic demanded by science ! We have given 
proofs over and over again of the course of normal infection ; these proofs 
are in accordance with the experience of others and in other parasitic Fungi. 
It may be quite true that certain possibilities can be conceived. Eriksson 
states (66, p. 143) that ‘if we try to make clear to ourselves the origin of 
the outbreak of an uredo-pustule fleck of an heteroecious species of rust (e.g. 
Puccinia graminis) we have to suppose several different possibilities. The 
fleck can arise (1) from an infection by uredospores (e.g. U redo graminis) \ 
or (2) from an infection by aecidiospores (e. g. Aecidium Berberidis). 
Numerous experiments have fully proved that both of these modes of 
origin occur. ... It is possible that an uredo-pustule can also arise : (3) from 
a direct infection by teleutospores (e.g. Puccinia graminis) without the 
intervention of an aecidium-stage ; or (4) from a latent germ of disease 
inherited from the parent plant. . . .’ 
Surely it does not need my emphasis of one of the fundamental rules 
in scientific reasoning — viz. that the mere assertion that such and such an 
idea or hypothesis is possible in no way supports its probability, and still 
less does reiteration of a hypothesis prove it ! Surely Eriksson must see 
that the omis of proof lies with him : not with us ! Until we know all 
about the infection by means of uredospores and aecidiospores — and recent 
work shows how much we have yet to learn as to the limits of germinating 
power, infective capacity, and so forth of the uredospores — we have no right 
