62 i'F:Rroi)iciT\' in organic jjfe. 



large proportion of the lambs born are twins. There are other 

 years when twins are few. I have never been able to account for 

 this ; and on looking back to the previous season, there seems to 

 have been nothing remarkable ; a fine summer may have been 

 followed by few twins, and a cold unfavourable season may have 

 had many. In the matter of food, so purely an artificial animal 

 kept under the conditions as the sheep in this country is, knows 

 little of scircity, and therefore this cannot much interfere with the 

 fruitfulness of the ewes. 



There is another domesticated animal which gives us an example 

 of the contrary result, and shows us how periodicity in scarcity may 

 be produced. 



During certain years, without any apparent difference in season 

 or food, it is noted throughout this kingdom that large numbers of 

 cows abort. This unfortunate accident is not confined to one farm 

 or one county, but is general throughout the kingdom. When a 

 season of this kind occurs, it must have a very considerable effect 

 on the numl)er of the young to be raised ; and, therefore, if these 

 animals were living in a state of nature, periodicity in scarcity would 

 be the result. 



Something of this kind may occur amongst creatures not under 

 man's control, but from want of opportunity we cannot say that it 

 does or does not do so. Arguing from what we see and know of 

 our domestic animals, it is, I think, a fair inference that possibly 

 this temporary fruitfulness or unfraitfulness may be one of the causes 

 contributing to the period of abundance or scarcity throughout all 

 nature. 



At various times for many years past, the Field Vole [Arvicola 

 agresfis) has become so numerous in the marshes of Essex, that the 

 whole of the grass has been eaten by them. The first mention of 

 this plague was in 1580. Since this time there are many records of 

 the destruction produced by these swarms of mice in these same 

 marshes. We now hear that the fields in the south of Scotland are 

 overrun with mice, to the serious detriment of the pasturage, and 

 that an application has been made to the Government for assistance 

 in ridding the district, which is a very large one, of these unwelcome 

 visitors or natives. ^Slany theories are promulgated to account for 

 the vast numbers existing ; amongst others that the destruction of 

 so-called vermin has, by doing away with nature's means of checking 

 undue increase, brought about this sad state of things. There 



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