Obviously there must be, in the formation of the soredia, great advantage for the species, or rather 'for the two species,' for the fungi as well as the algæ benefit by the arrangement, which ensures the continuance of the partnership. It was not without reason, however, that the dual organism was so long regarded as a simple species in the natural history sense, _for that is what it really is_, although it has arisen in a manner quite different from the usual origin of species. As we know species which consist only of single cells, and others which consist of many cells, differentiated in different ways, and forming a cell-community or 'person,' and, finally, others which consist of a community of diversely differentiated personæ, making up a 'stock'; so in the lichens we see that even different species may combine to form a new physiological whole, a vital unit, an individual of the highest order. When, at the outset of these lectures, I said that the theory of evolution was now no longer a mere hypothesis, and that its general truth could no longer be doubted by any one acquainted with the facts available, I had in my mind, among other facts, especially that of symbiosis, and above all the case of the lichens. There are many other interesting cases of symbiosis between two different kinds of plants, and one side of the partnership is represented by fungi in a relatively large number of instances. The reason is not far to seek: fungi must always be dependent on other plants for their food; they must be parasitic, because they cannot themselves produce the organic substances they require. They must therefore associate themselves in some way with other organisms, living or dead, and as a general rule they simply prey upon their associate, sucking up its juices and killing it. But in not a few cases they can render services in return, and, as we have seen in the case of the lichens, symbiosis may then occur. Fungi in general have the power of discovering and absorbing the least trace of water in the soil, and with it they absorb the salts necessary to the plant, and in this, apparently, consists the service which they are able to render even to large plants fixed deep in the earth, such as shrubs and trees. The roots of many of our forest trees, e.g. beech, oak, fir, silver poplar, and bushes like broom, heaths, and rhododendrons, are thickly wrapped round with a network of fungoid threads, and the mutual relations just indicated exist between these and the plants in question (Fig. 39, _A_ and _B_). The plants give to the fungi some contribution from the superfluity of their food-stuffs, and receive in return water and salts, which are of value especially in times of drought. Perhaps there is some connexion between this and the fact that limes wither and lose their leaves so quickly during great summer-heat; these and many other of our trees possess no root-fungi or mycorhizæ.