40 THE VEGETATION OF SMALL ISLETS. 



Darwin found that a piece of turf less than four feet 

 square, which had been exposed for many years to exactly 

 the same conditions, supported 20 species of plants, and these 

 20 species were found to belong to 18 genera and 8 orders : 

 which shows how much the plants differed from each other. 

 As we shall see presently the forms occupying a very small 

 and sharply defined space often belong to as many genera 

 as there are species. 



In this little archipelago of ours there are to be found 

 green islets of various sizes. Some of them are nothing 

 more than mere rocks capped with a modicum of vegetation. 

 Others are comparatively large, like Lihou on the west coast 

 of Guernsey, and Brechou, close to Sark ; each of which 

 possesses several acres of cultivated and pasture land, besides 

 of course, a large extent of rocky, infertile ground. When- 

 ever land is brought under cultivation, a new element is 

 introduced which modifies and alters the character of a flora. 

 Among the farmer's crops agricultural weeds spring up, the 

 tilled soil favours their growth, they increase and spread with 

 astonishing rapidity, and as a natural result in a very short 

 time a number of plants quite alien to the region flourish 

 among the truly native inhabitants of the soil. The residence 

 of a single family for even a twelvemonth upon some hitherto 

 uninhabited islet — assuming that a portion of the land is 

 worked and planted, and a few fowls and small domestic 

 animals are kept — is quite sufficient to effect a complete 

 change in its aspect, by the unintentional introduction of 

 numbers of agrarian weeds. I shall have occasion to give 

 a striking instance of this in speaking of the islet of Burhou. 

 But an expert botanist can always, or almost always, tell with 

 certainty by glancing at the plants that grow on a deserted 

 island whether it has in times past been inhabited or not. 



Many of the little verdure-capped rocks in our seas 

 are practically inaccessible. Sometimes the powerful current 

 and swell of the waves is too strong to permit of the approach 

 of a boat except on rare occasions ; sometimes the smooth 

 walls of rock that rise vertically out of the water afford 

 no foothold for an adventurous climber ; sometimes the land- 

 ing appears too hazardous to tempt even an enthusiastic 

 botanist. Several of the green islets within our area, how- 

 ever, can be reached without much difficulty ; and they 

 afford plenty of material for study to those who feel in- 

 terested in this line of investigation. 



Ever since I first began, some twenty years ago, to 

 pay any attention to the botany of the Sarnian Islands, 



