64 PERIODICITY IN ORGANIC LIFE. 



follow their usual habits, gradually and persistently moving onwards 

 in one direction, never returning to the place from whence they 

 started. What does all this point to ? Simply this — that the species 

 has arrived at its period of greatest abundance, and that Nature is 

 about to put a check on its increase, this being one of the ways in 

 which the period of scarcity is produced in this species, just as the 

 same migratory habits produce a similar effect in the locust. It 

 would appear that it is not the want of food which brings the migra- 

 tion of the Lemming about. The only thing we know is that they 

 appear in greater numbers than usual, and then are noticed to be 

 moving in one direction, the horde being increased by the numbers 

 which join it from every district through v/hich the band of 

 Lemmings passes. If the gathering is large, of course, like the 

 swarms of locusts, the animals must do much damage by destroying 

 all the food of the district in their line of march. 



I think from the illustrations I have given, that you all will be 

 able to understand what I mean by the term " periodicity." I think 

 that the examples given prove that when abnormal numbers of any 

 animal or plant occur, we may expect to see this abundance followed 

 by a period of scarcity, and that this periodic range of maximum and 

 minimum numbers is the result of a natural law controlling every 

 organized being, and is an effect not necessarily brought about by 

 man's agency, nor by climatic changes, nor variation in the supplies 

 of food. 



Applying the same law to diseases, we must not suppose because 

 the Black-death and other plagues of the Middle Ages have not 

 appeared for many years that they are necessarily extinct. They 

 probably exist, but at their minimum period; and we may find at any 

 moment that one of these dreadful scourges has started on its career 

 of destruction. It maybe that the increase of these diseases will be 

 the mode by which the enormous increase of the human race will 

 b? checked, an increase which, at a not very distant period, threatens 

 to so overcrowd the world as to make it a serious problem how all 

 will be fed. If there is any truth in the idea that there is a law of 

 periodicity we may make ourselves very easy, feeling sure that in the 

 armoury of Nature there is some beneficent law which will prevent 

 the dire results dreaded by many. 



