FUNGI. 409 
disease. History points out a singular correspond- 
ence between the dates of several of the great 
national epidemics of the middle ages and the as- 
certained period of maximum sun-spots, which is 
between eleven and twelve years. And as this 
period is accompanied by electrical and other dis- 
turbances in the earth’s atmosphere, it is highly 
probable that it may be connected with the potato- 
disease, which manifests a similar periodicity. The 
year 1872 was near the maximum of sun-spots ; 
and that year witnessed the most virulent out- 
break of the potato-disease since 1846; and just 
as the epidemics of the middle ages were associ- 
ated with cattle-disease and general failure of 
crops, so the potato-murrain of 1872 was associ- 
ated with epidemic diseases in animals and in 
field and garden crops. That season was one of 
the most unfavourable within this generation as 
regards the fruits of the earth. No certain pre- 
ventive of this destructive disease has yet been 
discovered, notwithstanding the many plans pro- 
posed, which fail as often as they succeed. 
I have said that the genus Peronospora, to 
which the potato-parasite belongs, contains several 
species which are often exceedingly destructive in 
this country. They are the most common and 
abundant of all fungi. For ages they have met 
the eye in fields and gardens. Onions, cabbages, 
