Now, whereas larch and spruce may often be found growing together in natural woods on the continent of Europe, it may be asked why the result of planting them together in British woods should be attended with such evil consequences. The explanation is to be found in the climatic conditions to which the larch is exposed in these islands. Naturally a mountain tree, in regions where a high summer temperature, long and strong sunshine, with little rainfall, but with much subterranean moisture from melting snow, promote vigorous growth, to be followed by total rest during severe winter weather, the larch meets in Britain with the reverse of these conditions--namely, a cool, cloudy, generally wet, summer, and an open and still wetter winter. The wonder is that the tree can adapt itself to the change as well as it does; there can be no doubt that its constitution does not remain so well able to resist attack by insect or fungoid parasite. Nature, which is ever as solicitous to provide for the perpetuation of what we consider ignoble vermin as she is for that of more admirable forms of life, has adapted the spruce-gall for a dual existence upon two species of tree growing in company; but she has also endowed these trees with a constitution vigorous enough not to suffer materially from the presence of the parasite. When that constitution becomes impaired by unnatural conditions of climate and environment, the parasite gets the upper hand, just as lice multiply upon a diseased bird or mammal. In the case of the larch, the mischief does not end with the aphides. Another enemy lies in wait for the tree that has been weakened by loss of its sap. A minute fungus (_Dasicypha calycina_), gaining access by its spores through any lesion of the bark, causes that incurable ill known as larch canker, which has now become so generally spread through British woodlands as to cause many landowners to give up planting larch at all. In this case, also, we have a parasite which may be found on larches in their native forests, but which the inherent vigour of the trees keeps in check. That this is the true and only reason for the excessive prevalence of larch canker in this country, causing incalculable pecuniary loss to many owners of woodland, is shown by the behaviour of the Japanese larch (_Larix leptolepis_). The fungus may easily be found upon this species; but so great is the vigour of the young trees that the fungus exists, and no more. The tree repels the inroads of mycelium into its tissues affording the invader foothold merely as a harmless guest.