VARIATION AND DISEASE. 169 



and the various species of American Vines in 

 Europe. 



These matters, in the hands of astute observers, 

 are turning the attention of cultivators and experts 

 to new aspects of the question of plant diseases, 

 namely, the possible existence of immunity, and 

 the breeding- of disease-proof varieties ; and the 

 existence on the part of the host plant of predis- 

 positions to disease which may depend on some 

 factors in the plant or in the environment over 

 which it is possible to exercise control, or which, if 

 known, can be avoided. 



The matter is complicated by the recent demon- 

 stration of the fact that parasites also vary and 

 can adapt themselves to altered conditions, as is 

 shown by the history of the coffee-leaf disease 

 {Hemileia) in Ceylon, and by Eriksson's results 

 with Wheat-rusts {Puccinia) and various experi- 

 ments with Coleosporium and other Uredineae ; but 

 there are good grounds for concluding that hybri- 

 disation, grafting, and selection of varieties may 

 do much towards the establishment of races which 

 will resist particular diseases, as shown by Mill- 

 ardet's experiments with Vines, and the results 

 obtained by Cobb and others with Wheat. 



The great difficulty with so-called " disease- 

 proof varieties " is to test them under similar con- 

 ditions in different countries, and for a sufficient 

 period of time. A particular race of Wheat may 

 behave very differently in Norfolk, Devonshire, and 

 Northumberland, and the recent introduction of the 

 purely experimental method in this connection is 



