268 HARRY MARSHALL WARD 



had no doubt that the bacteroids were the channel of supply. 

 But he failed to get any proof that they could assimilate free 

 nitrogen outside the plant. He suggested that their symbiosis 

 might be an essential condition, and was obliged finally to leave 

 it an open question whether the cells of the tubercles or the 

 bacteroids were the active agents in nitrogen assimilation. He 

 had already stated in 1887 that it is very probable that the 

 bacteroids " may be of extreme importance in agriculture." But 

 he was never satisfied with anything short of the strictest proof. 



In 1890 Ward was invited to deliver the Croonian Lecture. 

 He chose for his subject the relation between host and parasite 

 in plant disease. He defined disease in its most generalised 

 form as " the outcome of a want of balance in the struggle for 

 existence." But the particular problem to which he addressed 

 himself was the way in which the balance is turned when one 

 organism is invaded by another. This is the most common type 

 of disease in plants and a not infrequent one in animals. The 

 first result reached was identical with that of Pasteur for the 

 latter ; the normal organism is intrinsically resistant to disease. 

 It is an immediate inference that natural selection would make 

 it so. Ward then discusses very clearly the physiological con- 

 ditions of susceptibility, which he shows to be a deviation from 

 the normal. He had already indicated this in the case of 

 Entyloma. The epidemic phase is reached when the environ- 

 ment is unfavourable to the host but not so or even favourable to 

 the parasite. He then attacks the more obscure case where 

 there is no obvious susceptibility. This, he finds, resolves itself 

 into a mere case of the struggle for existence : " a struggle 

 between the hypha of the fungus and the cells of the host." It 

 is more subtle in its operation but of the same order of ruthless- 

 ness as the ravages of a carnivore. Ward's account of the 

 struggle is almost dramatic. The cellulose " outworks " are first 

 broken down, as he had previously shown, by a secreted ferment. 

 The " real tug of war " comes when the hypha is face to face 

 with the ectoplasm. Its resistance is at once overcome by 

 flooding it with a poison, probably oxalic acid. 



War with attack and defence is a product of evolution. 

 How did it come about in this particular case? Ward con- 



