2 ;6 HARRY MARSHALL WARD 



In such circumstances it might seem that the host was not 

 merely incapable of resisting invasion by the parasite but 

 actually invited its attack. Nature is, however, not easily baffled 

 in the struggle for existence. Attack provokes new methods of 

 defence. Ward soon found himself face to face with " problems 

 of great complexity," and these occupied the closing years of 

 his life. 



It had been ascertained in fact that the rust fungus is not, as 

 was at first supposed, a single organism, but comprises, accord- 

 ing to Eriksson, thirteen distinct species, each with physiological 

 varieties, and that those which are destructive to some grasses 

 and cereals, are incapable of attacking others. This necessitated 

 a scrutiny of the nature of grass-immunity. In a paper com- 

 municated to the Cambridge Philosophical Society in 1902, 

 Ward announced a conclusion which was as important as it was 

 unexpected. He had more and more made use of the graphical 

 method for presenting to the eye at a glance the result of a mass 

 of separate observations. In this case he uses it with striking 

 effect. He shows conclusively, as far as rust in brome-grasses 

 is concerned, that : " The capacity for infection, or for resistance 

 to infection, is independent of the anatomical structure of the 

 leaf, and must depend on some other internal factor or factors 

 in the plant." 



Finally, he is led to the conclusion that " it is in the domain 

 of the invisible biological properties of the living cell that we 

 must expect the phenomena to reside." He pointed out the 

 probability that light would be thrown on this from the action 

 of chemotaxis, on the one hand, and from that of toxins and 

 antitoxins in animal organisms on the other. This is a most 

 fertile conception, which would, however, have required a good 

 deal of verification, and this, unhappily, he did not live to 

 attempt. But with characteristic ingenuity he pointed out the 

 analogy between the infective capacity of uredospores and the 

 prepotency of pollen, which had previously engaged the attention 

 of Darwin. In a paper published in the following year in the 

 Berlin Annales Mycologici, he announced a no less significant 

 result. With his usual thoroughness in research he had cultivated 

 side by side at Cambridge more than two hundred species and 



