436 Prof. H. Marshall Ward. The Relations between 



In a day or two the infected circular areas, as marked by the brown 

 colour, were large enough to measure in millimetres, and by measure- 

 ments on successive days he was able to judge of the progress of this 

 extraordinary race, and he comes to the conclusion that on the same 

 substratum the conidia vary, according to their generation, in their 

 power of destroying the tissues. Those of the third generation, for 

 instance, not only germinate more rapidly and vigorously — indications 

 that they start in the race better equipped in the matter of food- 

 materials and ferment-yielding substances — but they also destroy the 

 cells of the host which they compete with more rapidly — which, no 

 doubt, indicates that they are able to produce or manufacture more 

 poison in the same time. 



Summary of the Factors of an Epidemic — Bearing of the Discussion on 

 other Parasitic Diseases — Conclusion. 



It will be clear from the foregoing that in the case of an epidemic 

 fungus disease, such as we have been considering, there are several 

 classes of factors to be regarded, and I may sum up the chief points 

 somewhat as follows. First, we have the normal healthy host-plant, 

 with all its hereditary (internal) and adaptive peculiarities ; secondly, 

 we have the parasitic fungus, also with its disposition. Then we 

 find, thirdly, that, apart from its inherent powers of variation, the host 

 is subject to variable external influences during its life, which may 

 produce such changes in the cell-walls and contents, &c, that the 

 plant approaches nearer and nearer the limits of health, wide as we 

 may regard these. On the other hand, we have, as a fourth considera- 

 tion, the parasite also varying under the influence of changes in the 

 factors of the environment, and its variations may, of course, be also 

 dangerous to its welfare ; but they may, on the contrary, be in such 

 directions that it is enabled to profit by the counter- variations of the 

 host. When the combined effects of the physical environment are 

 unfavourable to the host, but not so or are even favourable to the 

 parasite, we find the disease assuming a more or less pronounced 

 epidemic character. 



It is not pretended that we have here a totally new idea, because it 

 has long been known that some organisms which bring about parasitic 

 diseases do vary in the intensity of their effects, and can be made to 

 do so artificially, and we know that some of the most brilliant results 

 in biology have been obtained in connexion with certain lower 

 organisms ; but I have simply sought to show some of the links in 

 the chain of causes and effects in the definite case of certain epidemic 

 diseases of plants produced by the parasitism of some of the more 

 highly developed ^ungi, and this, I think, has not been done before. 

 If the preceding argument is admissible, new light will be thrown 



