54 PERIODICITY IN ORGANIC LIFE. 



that bad sanitary surroundings encouraged and made this disease 

 epidemic. If so, why did it not spread over the world ages before ? 

 There was the same neglectfuhiess of sanitary precautions, and all 

 unsanitary dwellings and surroundings had not disappeared when the 

 extreme virulence of the disorder had abated. The same may be 

 said of the disease I propose next to mention, and, in fact, of any 

 epidemic. 



The phenomena attending the appearance and disappearance of 

 other epidemics might easily be given, and the horrible disease, 

 Cholera, now, I fear, approaching our shores, affords another 

 excellent example of periodicity in abundance and decline. For 

 ages Cholera existed in India, but it did not put on the virulent 

 epidemic form which overran the world until about 1830. Then, 

 after a great increase for several years in India, an approaching 

 prosperity as I call it, started on its progress of destruction through- 

 out the world, killing all, or the greater part, of those it attacked, 

 and after a time following the usual course, diminishing in virulence, 

 until finally it disappeared from Europe. But it still went on in its 

 weaker form in India, until its period of abundance gradually 

 approached, and it then again spread over the world. 



These two diseases are as good examples of what I have called 

 " Periodicity in Organic Life " as could possibly be wished for. 



In plant hfe generally, the same periodical abundance of species 

 may be noticed. I do not wish any one to infer that there is any 

 regularity in the intervals between the periods ; there is nothing of 

 the kind. The times of prosperity in the species are most uncertain, 

 and cannot always be explained, as I have said before, by either hot 

 seasons, wet years, or any apparent surrounding causes. 



The first illustration I will take shall be from the lower plants, the 

 Fungi. Some of us remember, and all must have heard of, the first 

 appearance of the Potato Disease. In 1848, or thereabout, a disease 

 appeared in the potatoes in many parts of the kingdom, arousing some 

 interest from its effects on, and the destruction of, the crops which it 

 attacked. The next year, over large areas, the whole potato crop wab 

 destroyed. In Ireland, where the people depended almost entirely 

 on the potato for their food, a famine was caused ; and throughout 

 Europe much distress, disease, and suffering. This was the time 

 I have spoken of as the " period of prosperity " in the species. In 

 the following years much damage also resulted, but the destruction 

 has been gradually diminishing since that time, and, let us hope, the 



