Here, then, we have exchange of services between associated plant-elements and animal-elements--a performance by them of different organic functions for the benefit of the aggregate which they unite to form. Hence, when these vegetal elements and animal elements are separately embodied in plants and animals, which profit by one another, we may still properly regard their respective lives as mutually-dependent organic functions, as said in the preceding section. We are enabled the better to see how the Earth’s Flora and Fauna, which are respectively accumulators of motion and expenders of motion, form mutually-dependent parts of a whole, and are in that sense integrated. And we shall be prepared to see how all other relations between organisms which make them subservient one to another, similarly constitute elements in a general integration of the organic world. § 314_d_. Another form of mutual dependence and consequently of integration is conspicuous--that which accompanied the progressive increase of size in organisms of the higher classes. We have but to contemplate the possibilities to see that life must necessarily have commenced with minute forms, and that the progress to larger ones must have been by small steps. For had creatures of appreciable sizes been the first to exist they would inevitably have disappeared from lack of food. Having no resource but to devour one another, they would quickly have brought life to an end. There must have been smaller types serving as prey for larger ones before these could continue to exist and to multiply: microbes affording food to infusoria, infusoria affording food to such sized creatures as the _Entomostraca_, these again supplying food to small fishes, such as loch-trout, and these last yielding to larger fishes masses sufficiently great for their needs: each higher grade requiring lower grades of appropriate bulk. It needs but to ask what would become of tigers if there were no mammals larger than mice, to see that the animal world is a linked assemblage, of which the connected members stand within certain ratios of mass; and that during the evolution of higher and larger types the linking of grades has become closer.