THE FACTORS OF AN EPIDEMIC. 155 



and the yield of wine reduced from half a million 

 hectolitres to 50,000 only. Meanwhile the 

 disease had spread to Italy, Germany, Madeira, 

 Portugal, and even to the Cape, though not in 

 epidemic form as in the Bordeaux centre whence 

 it spread. 



Now it appears that Phylloxera has long been 

 in the habit of doing damage to vines in America, 

 where, however, it attacks the leaves, on which it 

 makes pocket-like galls, rather than the roots. 

 Moreover, there are species and varieties of 

 American vines which, even when planted in 

 Europe, do not suffer at all from this insect at the 

 roots, either because the rootlets do not push out 

 at the same season as those of the European form, 

 or because they form wood more rapidly and com- 

 pletely, or secrete resinous and other matters 

 distasteful to the insect in greater quantity and are 

 thus capable of healing the wounds, or in some 

 other way they do not respond to the attack or 

 suit the insect. In any case the attack on the 

 leaf rather than the root seems to be the exception 

 in European vineyards and the rule in American 

 species, and we appear to be face to face with a 

 problem of specific predisposition to this particular 

 malady. That the resistant properties of the vines 

 of America not all, only particular species and 

 varieties are thus "immune" can be utilised has 

 been proved by European growers ; and not onh' 

 so, for Millardet and others have shown that the 

 European vine grafted on to these resistant stocks 

 suffer less than when on their own roots. It has 



