Having arrived at the cell with its powers of division, the next step was the power of conjugation between cells with their interchange of vital substance, bringing about, in Weismann’s words, “a wealth and diversity of organic architecture which without it would have been unattainable.” It takes place by means of physical energies, but the process is entirely inexplicable unless we assume that it exists to satisfy a need, a _Drang_, for life. And this need, although of course it displays itself in physical processes, is not in itself a physical process. At the very beginnings of structural life, if not before it, we are obliged to pass beyond physics in order to comprehend physical phenomena. Whenever we find an aggregate of living units, such as a _Pandorina_ colony, living with a communal life which is other than the sum-total of the lives of the individual units, we are in presence at once of the necessity for a metaphysical conception, to render intelligible the unity in diversity which we perceive. The response of living protoplasm to the stimuli it receives from the outside world is normally directed to the maintenance of the life and form of the organism. The response of what is called ‘lifeless’ matter is of another nature; not because it is really lifeless, for if it were it would not respond at all, but because it has no organisms to protect and foster. We all know the nature of the action of gravity on Newton’s apple. It was treated as a dead substance, like a stone, and gravity acted upon it as upon all other ponderable matter. But when it had fallen to the earth, had decayed, and one of its pips began to grow, the action of gravity began to be manifested in a quite different and very peculiar fashion. It has been ascertained by a series of ingenious experiments that gravity is the force which obliges the roots of a plant to sink downwards into the earth. This does not, of course, mean that the roots are drawn downwards by attraction of the earth, but that the pull of gravitation gives a certain stimulus to the cells concerned which makes them grow in that direction. Precisely the same stimulus communicated to the cells of the stem has the very opposite effect—these it causes to grow upright into the air and light. Thus the roots are, as it is termed, positively, and the stems negatively, geotropic. The substance of the root cells and of the stem cells is the same, the stimulus is the same, but the effects on growth agree in only one point, that they are respectively what the plant requires them to be. There is no doubt that if a species of plants were placed in such a position that it would serve them for the roots to grow upwards, then upward-growing roots would eventually be evolved; in fact, this is actually the case in the lateral underground roots of certain mangroves which rise to the surface and become modified as breathing organs, and in the aerial roots of various orchids, etc.[52]