Introduction

Endemics are taxa which occur only in a limited area, for example in the British Isles or in Fennoscandia. Disjuncts are taxa distributed in different areas with considerable gaps in between so that genetic exchange is impossible between the separated populations. Centric distribution relates to limited areas with an extraordinary concentration of endemics and/or disjuncts.

The problems of interpreting endemic or disjunct distribution patterns are, in many ways, very different from the problems of understanding the distribution patterns of the more widespread taxa dealt with previously. In the latter case it is assumed that the distributional area is not limited by the ability of the taxon to spread and that the distribution is therefore limited by environmental factors. This assumption is supported by fossil evidence. Such an assumption cannot be made with respect to endemics and disjuncts. Instead it seems more probable that an explanation of their distributions is to be sought in the history of the populations, namely that they are relicts from earlier, different distribution patterns in the past when the plants could migrate under ecological conditions different from those of today. Such hypotheses should, however, be supported by fossil evidence (Godwin 1975).

In Chapter 4 the history of the flora was outlined based on fossils and geological evidence. The important events to which the flora had to adapt were the Pleistocene glaciations, when ice sheets covered most of Fennoscandia and the British Isles.