One of the principal obstacles to a fuller understanding of the history of the British flora in recent times is our ignorance of the precise effect on it of the Quaternary glaciations. We do not know with any certainty how many of the present British species survived glaciation from the Tertiary or from one of the inter-Glacial periods within the British Isles and how many are post-Glacial immigrants. There is an extensive literature bearing on the problem (Wilmott, 1930; Royal Society, 1935; Harrison, 1939, 1941; British Ecological Society discussion summarized by Tallantire & Walters, 1947, etc.), but the main question remains unanswered. The evidence in favour of survival is largely indirect, depending principally on features of present distribution difficult to explain in any other way (especially on the occurrence of two or more rare species together in widely separated, often small,* areas within or near the limits of glaciation, not wholly explicable by specialized ecological requirements) and on analogy with the climate and vegetation of existing glaciated areas. Botanical arguments against survival emphasize efficiency of dispersal and seek to explain association of rare species on the -basis of similarity of specialized ecological requirements yet to be discovered. Failure of such species to spread from the 'survival' areas is frequently advanced in support of this latter contention. This may perhaps be explained in part by the known loss of genetic variability in small populations with consequent loss of ability to adapt to new habitats (cf. Wright, 1940, etc.). Geological evidence on conditions during glaciation is on the whole unfavourable to the possibility of extensive survival through all the phases of the Glacial period. Fossil evidence is meagre and unsatisfactory, especially as far as the bryophytes are concerned, but extension of pollen analysis methods to inter-Glacial, late-Glacial and early post-Glacial deposits is giving results of great interest. Godwin (1949) has recently summarized some of the results obtained by workers at Cambridge. He shows that some of the species with a very limited present-day distribution in the British Isles had a much wider distribution in the pre-forestal conditions of early post-Glacial times, e.g. Potentitla fruticosa L. now known only from Teesdale and the west of Ireland, has been obtained from deposits in the Lea valley (near London). The last period of widespread distribution of some at least of the 'relict' species thus appears to have been post-Glacial, not Tertiary or inter-Glacial, and the subsequent fragmentation of range a concomitant of post-Glacial climatic changes. Up to the present, discussion of survival has centred on the higher plants. No attempt has been made to consider the sum of the evidence available from either British mosses or hepatics, though features of the distribution of certain species or groups have been discussed. Nicholson (1930), in working out a collection of hepatics from Yunnan, recorded