At first appearance, the ogham seem to be a phenomenon of the Celtic isles: the names are in the Irish language (and many are still in use for the same trees), and virtually all ogham inscriptions are from the Ireland or Britain (although claims have been made for inscriptions in northern Spain and North America). Graves (1966) postulated a much older origin, and his evidence, much of which is disputed by classical scholars, would tend to link the ogham to cultures of the eastern Mediterranean. Others have attempted to link the ogham to the Goddess-centered Old Europeans.
Linguistic evidence - There are three cognates between the ogham names and the English names for the same trees: beith, birch; sail, sallow (an archaic name for willow); iodhadh, yew. The scientific names were based on the old Roman names of these plants, giving a few more cognates: beith, Betula; sail, Salix; coll, Corylus; úr; Erica. Two of the ogham trees are also found in the Germanic runic alphabet: the birch (berkana) and the yew (eihwaz). Many of these tree names can be traced in other Indo-European languages, as well. (Friedrich  discusses Indo-European tree names in great detail.) So initially it would seem that the ogham have an Indo-European origin.
Botanical evidence - Nevertheless, archaeological evidence points to the Indo-Europeans originating in the steppes north of the Black and Caspian seas. Trees were very uncommon in these grasslands. They might have seemed even more magical for that reason, but most of the ogham trees do not occur there.
Where do they occur? Tutin et al. (1964) categorize the European flora in geographic areas that correspond to the countries of Europe in the 1960s (this is not scientifically sound, since plants don't obey national boundaries, but it was undoubtedly politically expedient). When we look at the entire list of twenty ogham plants plus common mistletoe, we find only four countries that have them all (Fig. 1): France, Germany, Switzerland, and Italy.
Britain lacks the silver fir and the grape (although the latter is cultivated), and Ireland lacks those plus the mistletoe. Old European civilization was well-developed in the Balkan peninsula, which lacks furze. Greece lacks birch, ash, oak, furze, and heather, and the eastern Mediterranean (outside the scope of Tutin et al.) lacks even more. If one were to pick a region where the plants of the ogham were best represented, it would be the valley of the Rhine River, home of the Iron Age La Tène culture that is regarded to be ancestral to the Celts.
If we substitute Quercus ilex for Ilex aquifolia as holly, and Loranthus europaeus for Viscum album as mistletoe, Italy is the only country with all the species, and Britain and Ireland have even fewer.
Thus, the traditional set of ogham trees seem to be resolutely Celtic, although not necessarily a heritage from the early Indo-Europeans. Evidence exists (from correspondences such as the holly and mistletoe above) for other, perhaps non-Celtic tree traditions in Europe. Whether these "alternate" trees had equivalent symbolic meanings is largely conjectural, although Graves has made a strong case for several.
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