loo DISEASE IN PLANTS. 



spores of a parasitic fungus which would infect 

 and rapidly destroy a potato plant in moist warm 

 weather may be showered on to such a plant 

 with impunity if the air remains dry and cool or 

 on to a cabbage under any circumstances as far 

 as we know. 



Again, probably no one factor of the non- 

 living environment ever suffices to induce a disease, 

 possibly because no such thing as only one change 

 at a time ever occurs. For instance, it is difficult 

 to say, when a soil becomes sodden with water, 

 whether the excess of water and dissolved matters, 

 the want of air displaced by the water, the lower- 

 ing of the temperature, or the accumulation of 

 foul products, etc., is the principal factor in causing 

 the damage which results, and we have to de- 

 termine by the balance of experimental evidence 

 which is the dominant factor in all such cases. 



The study of aetiology of disease is in fact 

 only a particular case of that of aetiology in 

 general. Plants at high altitudes in the Alps 

 acquire very different characteristics from the 

 same species in the plains. Is this due to the 

 low temperature, the rarer atmosphere, the more 

 intense illumination, the changes in moisture, etc., 

 etc. ? The question is more difficult than it 

 appears at first sight, and we must remember that, 

 complex as are the factors working on the host, 

 they are equally complex in their actions on a 

 parasite attacking the host, whence the resulting 

 disease becomes indeed a tangled problem of 

 natural selection. 



