154 BIOLOGICAL PROBLEMS 



could be added for large orchards and for other pur- 

 poses to fulfil local requirements. 



In a campaign it is important to know the disposi- 

 tion of the enemy, habits, movements, &c. ; so in a 

 campaign against disease we should possess full 

 knowledge of the organisms influencing the appear- 

 ance and spread of an epidemic and the conditions 

 causing it. Malaria could not be controlled until we 

 understood that the mosquito acted as a carrier, nor 

 typhoid until we had discovered the different types of 

 bacilli found in enteric fever. In the case of potato 

 blight, if Phytophthora infestans can only pass the 

 winter in diseased tubers, it should be possible to 

 eliminate the malady by selection of sets. But are 

 we sure that this is the only method of wintering? 

 Phytophthora can be grown easily on artificial media, 

 and, if so, may be capable of existing saprophytically 

 in the soil. Again, if potato disease appears in early 

 summer at only a few centres it would be possible to 

 isolate the outbreak and control it by the "ring- 

 fence ' method ; but if, on the other hand, there is 

 a simultaneous outbreak at a large number of places 

 it is important to spray wide areas. From the evi- 

 dence at our disposal, disease is most severe in Ire- 

 land, the south-western peninsula, western Wales, 

 and south-western Scotland, regions more or less 

 under the direct influence of the Gulf Stream. The 

 earliest outbreaks in 1917 occurred in the Isle of 

 Wight, Fishguard, and the Penzance district, in 

 coastal districts of low elevation and subject to sea 

 mists and fogs. The direction of the development of 

 the disease appears to have been roughly fan-wise 

 towards the east, north-east, and north. It corre- 

 sponds in a remarkable way with the progress of the 

 great murrain in Scotland. In 1845 the disease was 

 first observed in the south-western counties Wig- 



