98. We may consider the organism under two aspects--that of Structure and that of Function. The latter has two broad divisions corresponding with the vegetal and animal lives; the one is Nutrient, the other Efficient. The one prepares and distributes Food, the other distributes Motion. Of course this separation is analytical. In reality the two are interblended; and although the neuro-muscular system is developed out of the nutritive system, it is no sooner developed than it plays its part as Instrument in the preparation and distribution of Aliment. This not being a treatise on Physiology, there can be no necessity for our here considering the properties and functions in detail. What is necessary to be said on Sensibility and Contractility will find its place in the course of future chapters; for the present we will confine ourselves to Evolution on account of its psychological, no less than its physiological, interest. CHAPTER V. EVOLUTION. 99. That organized substance has the property of nourishing itself by assimilating from its internal medium substances there present in an unorganized state, and that this is followed by a development or differentiation of structure, is familiar to every inquirer. Every one who has pursued embryological researches, and in a lesser degree every one who has merely read about them, must have been impressed by this marvel of marvels: an exceedingly minute portion of living matter, so simple in aspect that a line will define it, passes by successive modifications into an organism so complex that a treatise is needed to describe it; not only do the cells in which the ovum and the spermatozoon originate, pass into a complex organism, reproducing the forms and features of the parents, and with these the constitutional peculiarities of the parents (their longevity, their diseases, their mental dispositions, nay, their very tricks and habits), but they may reproduce the form and features, the dispositions and diseases, of a grandfather or great-grandfather, which had lain dormant in the father or mother. Consider for an instant what this implies. A microscopic cell of albuminous compounds, wholly without trace of organs, not appreciably distinguishable from millions of other cells, does nevertheless contain within it the “possibilities” of an organism so complex and so special as that of a Newton or a Napoleon. If ever there was a case when the famous Aristotelian notion of a “potential existence” seemed justified, assuredly it is this. And although we can only by a fallacy maintain the oak to be _contained_ in the acorn, or the animal contained in the ovum, the fallacy is so natural, and indeed so difficult of escape, that there is no ground for surprise when physiologists, on first learning something of development, were found maintaining that the perfect organism existed already in the ovum, having all its lineaments in miniature, and only growing into visible dimensions through the successive stages of