638 



we are presented with a more complex and difficult problem. As we 

 descend in the scale of organised beings we find them more influenced 

 by external causes, their growth, nutrition, and reproduction are influ- 

 enced by causes, which possess no power over the similar functions 

 of beings higher in the scale of creation. But even the highest are 

 still more or less under the influence of external causes, some of 

 which we have no power to explain, foresee, or prevent. At intervals 

 we find countries ravaged by epidemic diseases, which do not appear 

 to be contagious, and are probably dependent on meterological causes. 

 Many animals are seen in great abundance in certain years, disappear- 

 ing and re-appearing at uncertain intervals. Locusts may be quoted 

 as an example. But if such causes operate on animals, how much 

 more may we expect that they should influence beings lower in the 

 scale! We consequently find that our amounts of agricultural pro- 

 duce vary very considerably and are dependant on the amount of 

 rain, dew, and other atmospheric causes. But this is more particu- 

 larly the case with the lowest orders of vegetation— the fungi. These 

 are often seen in large quantities one year, almost disappearing the 

 next, and recurring in a perodical abundance. 



What are the known causes of their variations ? Why should 

 epidemical disease, insects and fungi, thus observe an irregular perio- 

 dicity in their abundance ? Such circumstances lead us to the belief 

 that they must depend on atmospheric influences ; that variations in 

 the density, in the humidity, in the electric state, or finally perhaps 

 in the intensity of the earth's magnetism, are capable of influencing 

 both animal and vegetable life, and of inducing diseased states of 

 organized beings. 



The variations of humidity, whether in the state of vapour or rain, 

 have been recognised from time immemorial as causes of variations in 

 the amount of agricultural produce, but the influence of electricity 

 and magnetism, or even the changes of intensity of these powerful 

 agents, have not been determined with any degree of accuracy. It is 

 possible then, and I put it forward as an hypothetical statement, 

 that these influences may have produced the death of some portions 

 of the cellular tissue of the leaves. If we have a cause for the disease 

 of the leaves we may readily account for the subsequent state of the 

 stem and tuber. The leaves being the digestive as well as the respi- 

 ratory organs of the plant, if these are extensively diseased or destroyed, 

 two important functions will be deranged, or diminished, an insuffi- 

 cient, or unhealthy sap will be returned to the stem, and this will suffer 

 in its turn. As the tuber is simply a cellular development or distension 



