374 THE PRESIDENTIAL ADDRESS. [Sess. Lxx1. 
At any rate, such work should be undertaken in any national 
forest department. But a far more important department of 
plant ‘pathology is that of diseases caused by vegetable or 
animal parasites, and there will be many opportunities of 
studying these in a State forest. Any man who could 
furnish a reliable means of producing a larch tree that 
would be immune to larch canker would be a national 
benefactor. In the classic researches of Marshall Ward it 
was shown that in fungoid diseases of plants the infection 
or power of resisting infection depends on whether the 
enzymes and toxins of the attacking fungus were powerful 
enough to overcome the anti-toxins and other resistant 
bodies in the plant attacked. By injury or exposure to 
certain vapours the vitality of otherwise resistant plants 
may be so reduced as to give the victory to the disease 
fungus. The growing of great colonies of one plant in 
crowded areas opens the way to many of the bacterial 
diseases of which we have the parallel in such diseases of 
civilisation in man as tuberculosis or cholera. The great 
problem of plant hygiene, therefore, is to discover those 
cultural and other conditions which secure races of disease- 
resisting or immune individuals. To do this and at the 
same time to secure good cropping varieties, is not always 
easy. For example, Vitis riparia and Vitis rupestris, though 
both resistant to the vine disease, phylloxera, yield very poor 
grapes. But by crossing Vitis vinifera, Millardet with them 
Mr. Lewton Brain mentions that hybrids have been obtained 
which resist both phylloxera and mildew. By somewhat 
similar methods several workers have produced disease- 
resisting strains of wheat. Of course, when a crop of any 
kind is attacked, recourse may legitimately be had to any 
feasible plan for checking the evil. But in this connection 
I am satisfied that there is far too great a tendency to 
resort to the indiscriminate use of poisonous insecticides and 
parasiticides. The use of these powerful substances is always 
attended with danger, and has, in a Jarge number of cases, 
led to very disastrous consequences. J am inclined to think 
their use is unscientific. Instead of a wholesale massacre of 
the enemy after he has entered the city, it would be more 
scientific to devise some adequate means for his exclusion, or 
to surround him with those elements which would render 
