PERIODICITY IN ORGANIC LIFE. 63 



may be some truth in this notion, but the destruction of owls, etc., 

 is probably not the only cause of the increase of the voles. In other 

 places they are increasing, for instance in Greece. Does strict game 

 preserving obtain in that kingdom, and are all hawks, owls, and other 

 mice-destroying animals exterminated there ? If they are not, then 

 we cannot say game preserving and the destruction of the carnivo- 

 rous birds and animals of Scotland has brought about the plague of 

 mice, as Greece is also suffering in a similar and probably more 

 extensive scale, from the same mischievous little beasts. 



Again, in 1580 I do not think any great progress had been made 

 in Essex towards reducing the numbers of the mice-eating birds and 

 animals; still the pest appeared, got gradually worse, and at last 

 began gradually again to diminish, until only about the normal 

 numbers remained. It may perhaps be well to try and diminish the 

 number of these creatures in Scotland, but I thirrk man will be able 

 to do but little in effecting this desirable object, and in my opinion 

 they might as well be left alone, as the laws which govern this 

 periodicity in abundance and scarcity will bring about most surely 

 their usual results, and only a proper number of these animals will 

 be found in the course of a few seasons. This diminution has 

 always occurred wherever the pest has prevailed.' 



I will take the next illustration from a foreign rodent, the 

 Lemming. At uncertain intervals these animals abound to a mar- 

 vellous extent, and then commences a migration, the details of which 

 are very extraordinary, but we need not go into these, except so far 

 as they throw light on our subject. 



It would appear that this animal is very common throughout the 

 Scandinavian peninsula, just as the \'ole is in this country. After 

 a period (which may be long or short) it is noticed that there occurs 

 a considerable increase in the animal's numbers. This multiplication 

 goes on for some time, and at last it is found that the Lemmings are 

 evidently moving on, it may be towards the west. Nothing will turn 

 them from their course ; even a river or a lake will not stem the 

 migration. It must not be supposed that they assemble and start 

 off all at one time, as the swallow does, and so complete their 

 journey as quickly as possible. It would rather appear that they 



I Since writing this paper, an article by the Editor has appeared in the "Zoologist" for 

 April, 1893, not only confirmntory of the views I have expressed as to the influence of mice-eating 

 birds and mammals, but also, what is perhaps more interesting, bearing out the idea previously 

 mentioned that probably a larger fecundity may have had some influence in producing their 

 abundance. The writer states that Art<icola agrestis normally produces from four to six young 

 .It a birth, but at the present time in the districts of .Scotland suffering from this plague of mice 

 litters of eight or ten are very common — the equivalent of " a good fall of lambs." 



