PERIODICITY IN ORCIANIC LIFE. 53 



niiiiibers ; when it would be an illustration of the phenomenon of 

 periodicity, to which I am now calling attention. 



Periodicity in abundance is not confined to any class in organic 

 life ; it occurs in the highest as well as in the lowest, and I propose 

 to illustrate my meaning by taking examples first from the lowest 

 forms of life and afterwards from the higher. 



Epidemic diseases are, in all classes of organic life, due to a 

 germ ; this theory is, I think, now universally acknowledged. The 

 disease germ, then, shall be our first example, and as there is one 

 disease which will enable me to make my ideas on this subject of 

 periodicity more plain and clear perhaps than any other, I will take 

 that first. 



For many years, ages almost, there existed in the neighbourhood 

 of Boulogne a disease which was known to the French physicians by 

 the name of " Diphtherite," and to many of our countrymen who 

 visited that district as " Boulogne Sore-throat." Possibly there 

 may have been cases in this country also, and perhaps elsewhere in 

 Europe, but if so, the cases were so few in number that they attracted 

 little or no attention. Here, then, we have this species of germ at 

 its rare or almost extinct period ; it existed, but it was not prolific. 

 In 1852 or thereabout cases began, though rarely, to occur in various 

 parts of Britain, but they did not spread, and never became epidemic. 

 (Gradually, however, more and more was heard of them — the germ 

 was approaching its period of activity — and at last, in 1858, 

 Diphtheria, for this was the disease, overran not only Europe but 

 the whole world. 



This was the period of the abundance of the Diphtheria germ, 

 but, happily for mankind, its period of rarity seems approaching. 

 It is not so virulent (although even now bad enough), nor is it so 

 spreading as in its period of prolificness and abundance. 



This disease also enables me to illustrate the fact that man's 

 proceedings neither caused the sudden increase in the number of 

 attacks, nor did he in any way diminish them, excepting that by 

 care, and the adoption of the teachings of sanitary science, he, to a 

 certain extent, protected some portions of the population, and in 

 individual cases placed the affected in a better position to resist the 

 ravages of the disease. But he certainly did not bring about the 

 diminished virulence which was so apparent after the disease had 

 existed for some time, any more than he can be said to have made 

 the disease epidemic in the first instance. It will no doubt be said 



