The social or military exigencies that led people to construct artificial islands would also lead them to take advantage of such natural ones as would be found most suitable, and we may reasonably infer that it is in the absence of the latter that the former would be resorted to. We have therefore no _prima facie_ grounds for dis-associating chronologically the artificially-formed islands of wood or stone, either from each other or from such natural islands as may furnish evidence of early occupancy. The great and primary object of the island-builder was the protection afforded by the surrounding lake or morass, the securing of which has continued to be a ruling principle in the erection of defensive works down to the Middle Ages, long after the wooden islands ceased to be constructed. The transition from an island fort to the massive mediæval castle, with its moat and drawbridge, is but another step in the progressive march of civilisation. When the greater advantages of stone buildings became generally recognised, the old wooden refuges, so liable to decay, so easily destroyed by fire, and so unsuitable for supporting heavy buildings of masonry, would be gradually superseded. It would then be found easier and better to conduct the water to the stronghold than to construct the stronghold in a natural basin of water, however convenient its locality might be. To the transitional period preceding this great change, which culminated in the almost impregnable moated and mediæval castle, may be assigned many of the remains of stone forts, castles, etc., still abundantly found in bogs, drained marshes, and natural or, maybe, artificially built islands. While it is therefore possible to assign the wooden islands to a fairly well-defined period, which, speaking generally, precedes that when stone and lime were used for building purposes in this country, the claims of all other island homes to great antiquity must be judged of by their special peculiarities. The annexed tabular statement comprises not only all the known artificial islands, whether constructed of wood or other materials, but also some natural ones known to have been artificially strengthened or fortified, as well as a few examples of other structural remains, such as camps, castles, etc., now or formerly located in bogs or lakes. The first column of numbers in this table contains only the crannogs proper, _i.e._ islands constructed on wood and surrounded by piles; while the second includes all the remaining classes. The characteristic or differential features of all these examples, when not referred to in the text, will be found in the marginal notes or references, so that a mere glance gives a general idea of their number, character, and geographical distribution. TABLE showing GEOGRAPHICAL DISTRIBUTION OF LAKE-DWELLINGS, ARTIFICIAL ISLANDS, ETC., IN SCOTLAND, with Notes and References. Those to which no reference is given will be found described in the text.