With a like effort in the attempt to obtain food, that organism will be best nourished which commands the best means of obtaining and preparing its food,--the best apparatus for the seizure and grinding of food, and the best salivary gland. And finally, greater surface of skin, of lungs, of gills, or of intestines, causes greater capacity for assimilation, and since this surface is increased by cell-division or propagation, the capacity of the organism for assimilation grows with its capacity of propagation.[59] Protoplasm is never entirely homogeneous, and we must suppose some difference even in the beginning; such difference is, indeed, fundamental through the very composition of protoplasm from the four fundamental elements, and this or that other element. These different elements must be held together by forces of attraction, and the direction of these forces must have some common centre represented by some differentiation of the protoplasm, whether as clearer spot, or as nucleus. This spontaneously generated organism, neither animal nor plant, is nourished, as we have seen, by diffusion, by the transformation of inorganic into organic substance. The lowest organisms possess no definite organs for taking food; they manifest, however, phenomena of movement which are exactly like those of the animal organism, for they appear unconditioned and hence voluntary. Locomotion is, in the lowest animal forms, the only means of obtaining nourishment. The amoeba surrounds and takes in whatever is by chance met with. Animals a little higher in the scale swim about and seek their food; or, remaining in one place, they cause, by means of cilia, a movement of the water towards a certain part of the body, a sort of mouth where the protoplasm is open and can take up the prey in the same manner as does the amoeba. Ascending the scale of life, we find more and more complicated apparatus for the seizure of food, for its preparation and digestion, and the beginning of a nervous system, first as the differentiation of certain muscle-cells, then in connection with a special sense, that of hearing. If we assume any pleasure to be connected with the earliest acts of assimilation, it must be that of the satisfaction of a want, the stilling of pain in the form of hunger. THE PROBLEM OF PERFECTIBILITY