All this activity of the separate cells and combinations of cells is taking place. And in addition there is that activity of them all in their togetherness, that activity which keeps the cells together, and which if relaxed for a moment would mean that the cells would all collapse as the grains of dust in an eddying dust-devil at a street corner collapse once the gust of wind which stirred them and keeps them together drops away. What must be the intensity of life required to develop the tree from the seed and to rear that giant straight up from the level soil 200 feet into the air and maintain it there two hundred years, we can only imagine; for to outward appearance the tree is quite impassive. It does not move a muscle of its face to reveal the intensity of life within. The tree is characteristic of every living thing. Every plant and every animal, however seemingly sluggish, is working to fulfil its life, to nourish itself, to reproduce its kind. * * * Now, the amount of air and sunshine for plants may be practically unlimited, but air and sunshine are not all that plants require. They want soil and moisture as well. And the standing-room for plants is strictly limited. The forest stretches away up to the snows; but there it stops. Necessarily, therefore, there must be the keenest and most incessant struggle among the plants for standing-room. Only a comparatively few can be accommodated. The rest cannot survive. And as the number of plants which can survive is thus limited, the number of animals is limited also, for animals are dependent on plants. Plants, therefore, in spite of their eminently pacific appearance are engaged in a fierce struggle with one another for standing-room. And animals are likewise engaged in a struggle among themselves for the plants. There is competition among the roots of the different individual plants for the food and water of the soil. And there is competition among the leaves for the sunlight. Each plant is pushing its roots downwards and spreading outward for more food and to root itself more firmly. Each is straining upward to receive more sunlight. Each is struggling with its fellows for room and means to develop its life. Competitors in hundreds and thousands are forced to withdraw and succumb. And even when a forest giant has defeated all competitors and reached its full maturity it has still to maintain the struggle and hold its own continually against other individuals whose roots are reaching out below and whose branches are spreading out above; against climbers who would smother it; and against parasites who would suck its very life-blood. The battle, moreover, is often not so much between one species and another species as between individuals of the same species. And it is a war which continues through life.