Again, the plant derives the material framework of its structure from the air, by breathing in through its leaves the carbonic dioxide present in the atmosphere, decomposing it, fixing the carbon in its roots, stem, and branches, and exhaling the oxygen. The animal exactly reverses the process, inhaling the oxygen of the air, combining it with the carbon of its food, and exhaling carbonic dioxide. Thus, a complete polarity is established, as we see in the aquarium, where plant and animal life balance each other, and the opposites live and thrive, where the existence of either would be impossible without the other. Sharp, however, as the contrast appears to be in the more specialised and developed specimens of the two worlds, we have here another instance of the difficulty of trusting to first impressions, and have to modify our conceptions greatly, if we trace animal and vegetable life up to their simplest forms and earliest origins. In the first place, each individual vegetable or animal begins its existence from a simple piece of pure protoplasm. This develops in the same way into a nucleated cell, by whose repeated subdivision the raw material is provided for both structures alike. The chief difference at this early stage is that the animal cells remain soft and naked, while those of vegetables secrete a comparatively solid cell-wall, which makes them less mobile and plastic. This gives greater rigidity to the frame and tissues of the plant, and prevents the development of the finer organs of sensation and other vital processes, which characterise the animal. But this is a difference of development only, and the origination of the future life from the speck of protoplasm is the same in both worlds. If, instead of looking at the origin of individuals, we trace back the various forms of animal and vegetable life from the more complex to the simpler forms, we find the distinctions between the two disappearing, until at last we arrive at a vanishing point where it is impossible to say whether the organism is an animal or a plant.