To secure life more simply is better than to secure the same amount by means of complex apparatus. Of course when the favouring conditions are altered, then any apparatus that makes life still possible is an advantage; but till that crisis arises it is only an encumbrance. When life can be secured only at the cost of greater labour and exertion and cunning, it is well to be capable of these things, but surely those animals are more to be envied that have no need of these things. It is only on the hypothesis of an unkindly environment that complexity of organization is an excellence. Furthermore, although these accidental variations allow certain creatures to survive in crises of difficulty, yet they also make the conditions of their survival more complicated and hard to secure. All that differentiates man from an amoeba has enabled him to get safe through certain straits where the lower forms of life were left behind to perish; but it has also made it impossible for him to live in the simpler conditions he has escaped from; like a parvenu whose luxurious habits have gradually created a number of new necessities for him, which make a return to his original poverty and hardships quite impracticable. If the development of lungs has allowed animals to come out of the water into the air, it has also prevented their going back again. Furthermore, a considerable amount of vital energy is consumed in the production, support, and repair of all this supplementary, life-preserving apparatus; just as, much of the national wealth for whose protection they exist is absorbed by a standing army and other military preparations. And in fact of two countries otherwise equal in wealth, that is surely the better off which has no need of being thus armed up to the teeth. Thus man's superior organization may be compared to the overcoat and umbrella with which one sets out on a threatening morning; very desirable should it rain, but a great nuisance should it clear up. It seems, then, that the highest organism is that which produces or secures the greatest quantity of life in the simplest manner, and at the cost of the least complexity of structure and function; while the lowest is that which yields the least quantity at the greatest cost; and between these two extremes organisms will be ranked by the ratio of their complexity to their life-productivity--life being measured mathematically (as something homogeneous) by its vigour, by its duration, and by the amount of matter animated, whether in the individual or in its progeny. It is obvious how, at this rate, our zoological hierarchy is turned topsy-turvy; and how difficult it will be to show that man is a better life-machine than, say, a mud-turtle with its centuries of vital existence.