No one, however well he may conceive the nature and history of the organic hosts of the earth, can hope to convey to the general reader an adequate sense of their majesty or the wonderful part they have played in the history of the life which has culminated in mankind. The largest words are freighted with too little meaning, and even the metaphors drawn from human associations fail to convey a sufficient picture of these enduring organizations which have enabled living beings to meet the difficulties of their long contest with this rude world, and to win the advance they have gained. The reader will have to tax his imagination to picture, it may be, a quarter of a million species dwelling in the same field, each united with the other in the method of exchange in such a way that the withdrawal of any one form is likely in some measure to change the estate of every other. In some cases this removal of one species means the loss of the life of many and perhaps the better opportunity of other neighbors; again, the influence on remoter members of the society may be so slight as to escape detection. Yet it is doubtful if the slightest change in the population of a biologic province can be brought about without some effect upon all the members of the society. It is a vast, sensitive thing, fit to be compared with the living body where every cell lives in accord with every other of the frame. So long as the organic hosts were in the prehuman stage the maintenance of the accord was easily and naturally attained. Species arose and perished, each in turn effecting a simple reconciliation with the others, grasping only so much room and food as was necessary for its proper support. But with the coming of man, the species which by its swiftly progressive desires has become a host in itself, a disturbing element was introduced into the old order. Man as a primitive savage falls into the natural system without greatly disturbing it; but man as a soil-tiller, in so far as he carries out his subjugative work, utterly wrecks the ancient establishments of life. To attain his object he has to banish from the soil nearly all the plants which originally belonged upon it, and in their place, with or without intention, he introduces species from other organic provinces. With the change in plant-life necessarily goes a like, or even a greater, alteration in the native animals. They are driven into the wilderness or, it may be, extirpated. The reader who would obtain an idea of these changes will do well to study the invasions of weeds or of those noxious insects which in the economy of a civilized country may be likened to weeds. These pests are in nearly all cases invaders which owe their successes to the fact that our treatment of the regions they have entered has opened vacancies in the once closed ranks of the indigenous host, into which the foreigners are free to enter. In the fresh field they are not