TEE ORIGIN OF FOREST GROUPINGS. 233 



Another distinct grouping of species in the bosom of this 

 region is dependent on the nature of the soil. It includes the 

 cork-oak, the chestnut, and the maritime pine, which are limited 

 to the siliceous zone, and with them a whole train of plants and 

 shrubs which are found with striking uniformity wherever the 

 mineral composition of the soil is of similar character. So strict 

 an adaptation, so absolute a selection, could not have been the 

 work of a small number of centuries, but they may properly be 

 attributed to causes that existed in a remote past. 



To complete our review of the Mediterranean forest-grouping, 

 we should consider, besides the dominant forms, some exception- 

 ally sheltered parts of the regions, and other regions in which 

 intermittent rigors of temperature have spared only the hardiest 

 types. Three elements may be distinguished in the aggregate of 

 about two hundred species included in the forestal vegetation of 

 this region: First, the principal and characteristic element, in 

 which plants with persistent leaves predominate, and which in- 

 cludes among its rarer types species in a declining condition, 

 which are cantoned upon the best-sheltered or most southern 

 points, and are transitional toward tropical types ; next, the 

 mountaineer element, to which altitude is favorable ; and a third 

 element, to which heat and moisture are agreeable. The last in- 

 cludes plants with deciduous leaves, which, while they can accom- 

 modate themselves to a cold climate, are better adapted to south- 

 ern temperatures, and do not grow spontaneously in the central 

 region ; with which corresponds a group of trees — large, not very 

 numerous, and usually monotypal — which deserves attention 

 because of what it has been in the past, and which may be said 

 to represent the southern prolongation of the central group, elbow- 

 ing into the Mediterranean region. Its members, including the 

 alder, the Eastern witch-elm, the hornbeam, the plane, the liquid- 

 ambar, the fig, vine, some ashes, lindens, and walnut, are more in 

 contrast with the mass of the Mediterranean plants than with the 

 types equivalent in order that are domiciled farther north. It 

 might be said of them that, while they are found associated with 

 the former plants, they belong naturally to the category of the 

 latter, as they would visibly had not the free extension of these 

 to the southward been arrested. This view is confirmed by the 

 examination of the forestal flora of a corresponding latitude in 

 America. The absence on that continent of any vegetable domain 

 equivalent to that of the Mediterranean disengages that element, 

 and supplies, through the plane, the liquidambar, the persimmon, 

 and American vines and walnuts direct from the ancient world, 

 a parallelism or a repetition of forms which the study of pale- 

 ontology helps to illustrate. 



The principal elements of the Mediterranean group — hollies. 



