Though ecology has been studied for a long period on the Continent it is only within the last ten years that it has become an important branch of botany in this country. Now, however, it is one of the most ardently pursued sciences, and has made great progress. That branch of it which deals with soil factors, or the distribution of plants upon different soils and a knowledge of the nativity of plants upon particular soils, has received considerable attention, and sufficient data have been accumulated to state the principle that each rock-soil supports a characteristic flora: so that the type of soil may be regarded as an index to the type of vegetation of any district where that rock-soil is dominant; and in the same way it is possible to state by a recognition of the type of vegetation the character of the rock-soil.