It is probable that these three stages of asexual reproduction are not all the steps actually taken by Nature in the development of the early life-process. There must have been intermediate steps, perhaps many such, but the forms in which they occur either have not persisted, or have not yet been studied.[11] The feature common to all ordinary forms of asexual multiplication is that the reproductive process is independent of sex; what starts the new life is the half, or a liberated portion of the single parent cell. It will be readily seen that by this process the offspring are identical with the parent. Life continues, but it continues unchanged. Thus the power of growth is restricted within extremely narrow limits. Any further development required a new process. With the life-force pushing in all directions every possible process would be tried. We are often met with striking phenomena of adjustments to new conditions, which in some cases, when found to be advantageous to the organism, persist. There is, in fact, abundant evidence that Nature in these early days of life was making experiments. In pursuance of this policy it naturally came about that any process by which the organism gained increased power of growth had the greater likelihood of survival. The number of devices in the way of modification of form and habit to secure advantage is practically infinite; but there was one principle that was eagerly seized upon at a very early stage, and, persisting by this law of advantage, was utilised by all progressive types as an accessory of success. This was the principle of fertilisation, which arose in this way from what would almost seem the chance union of two cells, at first alike, but afterwards more and more highly differentiated, and from whose primordial mating have proceeded by a natural series of ascending steps all the developed forms of sex. The ways in which this was brought about we have now to see. But even at this point it becomes evident that the true office of sex was not the first need of securing reproduction--that had been done already--rather it was the improving and perfecting of the single-cell process by introducing variation through the commingling of the ancestral hereditary elements of two parents, and, by means of such variations, the production of new and higher forms of life--in fact, progress by the mighty dynamic of sex.[12]