Harry Marshall Ward. vu 
War with attack and defence is a product of evolution. How did it come 
about in this particular case? Ward convincingly traces out the whole process. 
The normal plant obtains its food from inorganic material. But when 
opportunity offers it easily lapses into a condition in which it takes the 
material for metabolism ready made from the decay of others and becomes 
saprophytic. Ward shows that it is only a step to the attack on the living, 
and for the saprophyte to become a parasite, and he further shows that it can 
be readily educated to be so. He does not hesitate to suggest that the 
function of conidia in the complicated cycle of fungal reproduction is to 
form the cellulose-dissolving ferment. But now and again the host does not 
succumb to its invader. A truce is sometimes called in the struggle, and 
host and parasite are content to live together in a mutually advantageous 
symbiosis or commensalism. 
Three years earlier, in 1887, Ward’s attention had been drawn by a happy 
accident to the physiological aspect of symbiosis, and it never ceased to 
occupy his mind. It was well known that ginger beer was made in villages 
in stone bottles. The fermentation was effected by the so-called “ ginger 
beer plant” which was passed on from family to family, but nothing was 
known as to how or where it originated. It seemed to have some analogy 
with the Kephir of the Caucasus. A specimen was sent to me from the 
Eastern Counties, and it stood for some time in the sun in my study. 
I noticed the vigorous growth accompanied by a copious evolution of gas. 
Ward coming to see me one day, I handed it over to him as a problem worth 
his attention. At the same time Prof. Bayley Balfour had examined it and 
concluded that it was a mixture of a yeast and a bacterium. Its study 
involved Ward in a very laborious research which occupied him for some 
years, and of which the results were published in the ‘ Phil. Trans.’ in 1892. 
It proved to be a mixture of very various organisms, every one of which 
Ward exhaustively studied. This required not less than 2000 separate 
cultures. The essential components proved to be, as Balfour had suggested, 
a yeast derived from the sugar and a bacterium from the ginger. Both were 
anaérobic ; the yeast fermented cane-sugar with the copious production of 
carbon dioxide but little alcohol; the bacterium also produced carbon 
dioxide, even in a vacuum tube. 
The action of the two components studied separately proved to be not the 
same as when they worked in concert. This was conspicuously the case 
with the evolution of carbon dioxide, which proceeded with such violence as 
to make the research attended with considerable danger. It is known that 
the action of ferments may be checked by the inhibition of the products 
formed. Ward pointed out that while the use of these might be advan- 
tageous to the bacterium, their consequent removal might be equally so to 
the yeast. This established the important principle of symbiotic fermentation 
and gave it a rational explanation. On the morphological side Ward showed 
that the ginger-beer plant is comparable to a gelatinous lichen, and, having 
resolved it into its constituents, successfully reconstituted it. 
