PERIODICITY IN ORGANIC LIFE. 55 



period of scarcity is approaching. The only thing man has done, ur 

 can do, apparently, to diminish the evil effects of the disease, has 

 been to cultivate those forms of the potato which have been proved 

 the best to resist attack. 



Has weather had any effect ? Certainly it has to some extent ; 

 heat and moisture always encourage the growth of fungi. But an 

 examination of the recorded temperatures of the disease years since 

 1848, will show that, as frequently as not, the bad years have been 

 cold as often as hot ones — in short, as far as we can see, meteoro- 

 logical effects have had little or no influence in promoting or 

 preventing the appearance of this disease. 



Some years since, that condition of certain corn and grass seeds 

 known as Ergot was very prevalent in Essex. It was the period of 

 prosperity of the Ergot fungus. 



Now we appear to be approaching the time of scarcity, and can 

 anyone say that man's proceedings favoured the abundance of the 

 fungus or that meteorological conditions reduced it ? The Ergot 

 has had its abundance in our county, and although now rare, it 

 certainly will appear again. 



Those who live in Essex will sometimes notice, it may be 

 for some years in succession, how very abundant the wild oat 

 {Avena fatud) is, in wheat or other crops, over large districts of the 

 county. 



Why is this ? Is it not an illustration of the same law, that 

 everything has its period of abundance and the contrary ? I may 

 be told that the wild oat is more common in wet years, but some 

 further explanation more than this is required, because I have 

 noticed them just as abundant during a succeeding dry year. 



Parasitic plants, like fungi and disease germs, are good examples 

 of periodicity. Who has not noticed the extreme abundance for a 

 few years of the Orobanch of the clover ( Orohaiiche minor) and then 

 its almost entire disappearance for a shorter or longer period. 



Meteorological effects seem here also to exert no influence, 

 and, as far as I know, no explanation can be given for its abundance. 

 We can only say it is undergoing, from some cause, a period of 

 prosperity. 



About 1844, there appeared in British waters a plant from 

 America (Anacharis alsinastrum) which could have been well spared. 

 It soon overran the whole kingdom and threatened to block up all 

 our more slowly flowing rivers and canals. 



