The countrywide selection of key sites has thus to be made within a range of ecosystems which has been reduced in total area, greatly fragmented in distribution, and often depleted in quality, through human influence, but which may actually have become expanded in diversity by the same token. There is a contrast between the lowlands of the south and east with a large number of relatively small, diverse and scattered sites, and the uplands of the north and west with large, continuous masses of semi-natural habitat with rather less diversity in relation to the size of areas involved; both situations pose complex problems in survey and assessment. The limits of our sphere of interest in nature conservation are difficult to define as habitat passes from the semi- natural into the obviously artificial, and decisions here have necessarily to be subjective and arbitrary. The foregoing brief account of human impact on the original field of variation emphasises the difficulties of assessing and selecting sites within an ecosystem complex which is constantly changing, usually by loss of interest and area, but occasionally by the creation of new habitats. Values in this sphere are indeed relative, and future modifications to one site may change both its rating and that of other related sites.