I^ HE botanising hitherto done in the Alps gives us but very imperfect information on the local distribution of the alpine flora. Attention has been specially directed to rare plants, whose least localities are recorded, while the common species are often neglected. But from the standpoint of the factors which regulate distribution the common species are the most important. The rare species of the alpine flora, those which appear only in a few isolated stations, sometimes only in a single one, are usually either species with a very sporadic general distribution, or they are endemic species, or finally they may be" glacial relicts." All these categories are of great interest from the standpoint of the history of floras; their presence in the stations which they now occupy is explained not only by the ecological conditions they find in these stations, but also by historical causes, and especially by the conditions of post-glacial immigration.^ This article is translated from the French original (which appeared in the Revue geiie'rale des Sciences, 15th December, 1907, pp. 961—967) and published in THE NEW PHYTOLOOIST (by kind permission of M. Olivier, the editor of the Revue ge'iie'rale) at Professor Jaccard's request. The Editor acceded to this request the more readily, inasmuch as the statistical method employed by Professor Jaccard appears worthy of being tested over a wider range of vegetation. With suitable development, these and similar statistical methods promise to form an important means of connecting the study of floristic distribution with that of the determination and distribution of units of vegetation.—EDITOR, NEW PHYT.