XXX VI INTRODUCTION. 



die from heat, from cold, from light, from darkness, 

 from electricity, from moisture, from dryness, from 

 improper soils. We isolate all these causes by finding* 

 that, under all variations of their influence, the disease 

 more or less appears ; we isolate the effects of vege- 

 table fungi, by showing that they do not appear till 

 the plant is damaged. 



With regard to animal parasites, we discover one 

 particular Aphis which comes before any part of the 

 plant exhibits the malady. But here, again, in the 

 potatoe plant, we have other sources of error. A 

 diseased plant propagates its disease ; hence we must 

 prove previous health before we can show that the 

 insect comes before the disease. 



We then look about other plants, and find that 

 they are also attacked by the same Aphis ; and that 

 when they are so attacked, the same effects are mani- 

 fested. The mode of death is also precisely similar 

 in these plants to that in the potatoe ; for, in all cases, 

 the collar of the plant becomes swollen with watery 

 matter and dies ; and thus the root is separated from 

 the leaf, and the entire plant perishes. 



In all these cases the vastator is the antecedent, 

 and the death of the plant the consequent, of the 

 malady. We desire naturally to inquire whether 

 this injurious action is confined to one species of 

 Aphis ; and we find large trees destroyed, and other 

 useful crops annihilated by other species. 



Here we have our argument strengthened by ana- 

 logical cases ; but we are naturally led to inquire why 



