laurels, olives, myrtles, etc.—with their narrow, elongated, coriaceous, entire or spiny foliage, only slightly divided, do not display the luxuriant fullness of tropical forms, but seem to lead toward them. They touch upon them on some sides, indicating the influence of a special medium, determined by conditions of intermittent heat and dryness. A peculiar feature of the Mediterranean group, and one which may help to determine its significance, is the capricious and uneven distribution in the interior region of plants of the most decided characteristics, evidently outside of the range of common species, manifesting affinities with hot country types. These species are narrowly cantoned in certain stations. The Pinus excelsa of the Himalayas is found only on a single mountain of Macedonia; the Algerine Thuya only in the Atlas; the false cork-oak in only a few specimens at a single spot; the carob at a few places on the littoral; the poplar of the Euphrates on the banks of the Jordan and at one point in the province of Constantine. Numerous instances of this kind betoken the existence of a former condition which has been more or less changed by subsequent events, that the group has suffered from revolutions which have displaced and partly eliminated elements that were formerly more widely distributed. The group has been impoverished, probably by the depreciation of some elements, certainly by the destruction of others.
This brief review of the forestal zones from north to south is sufficient for the study we have in view of the paleontological origin of the principal types of trees. This origin, which is at the best hard to determine, could not be sought with any probability, except as to those species concerning which we have data of a character to cast light on their history in the past, their former migrations, and their career through time as well as through space. Europe, North America, and the arctic zone furnish these data. They are not to be found in India, China, and Australia, for the guiding thread would be wanting there. Knowledge of many fossils is not enough of itself to conduct to the desired end. Vegetable impressions are useful indications, which, taken singly, have a relative value, but rarely lead to results of a material bearing.
Multiplied observations and discoveries, and fossil beds of unusual richness, to be explored at many points from north to south, have been required to give a view of the floral past of a part of the globe. It was also necessary that these beds, instead of belonging to a single period, should be frequently separated by long intervals distributed through successive ages, so as to present the complete picture of the series of past times. Thus a comprehensive grasp has become possible of the vicissitudes through which the vegetable kingdom has gradually undergone transformation.