That among plants considered as an aggregate relations of like kind, though far less distinct ones, have all along been growing may be reasonably concluded. In a world peopled only by microscopic types there could not have existed the conditions needful for large trees. Gradual disintegration of rock-surfaces, partly effected by physical agencies and partly by low forms of plants, had to prepare the way for superior plants. The production of sufficient soil by mineralogical decay as well as by the decay of organisms, plant and animal, may be regarded as having been a preliminary to larger plant-growth; and though at present the dependence is far less close than that among animals, yet the benefits yielded to metaphytes by the decomposing actions carried on by protophytes, as well as those carried on by microbes permeating the soil, imply a continued general interdependence throughout the aggregate of plant-forms, apart from more special interdependences. And then along with this indebtedness of the greater plants to the smaller during the process of evolution, there must be named that indebtedness of plant-life to animal-life which Mr. Darwin has shown in his book on the agency of worms as producers of mould. § 314_e_. Services of one to another, and consequent unions, of more special kinds are infinitely varied, alike within each kingdom and between the two kingdoms. I refer to those seen in parasitism, commensalism, and other forms of association. While they do not conduce to unions of the kind thus far considered, these nevertheless constitute innumerable links whereby the lives of organisms, plant and animal, are tied together; sometimes for the advantage of both but in most cases for the benefit of one to the injury of the other.