There is, however, a test, the most reliable of all that have been discovered, by which an animal may be distinguished from a plant, and that is the nature of the food and the products which are elaborated therefrom in the body. Plants live upon such inorganic substances as water, carbonic acid and ammonia, and they have the power of manufacturing out of these true organic materials, and are therefore the great producers of nature. All plants which contain green coloring matter, technically called chlorophyll, break up carbonic acid in the process of digestion into its two constituents of carbon and oxygen, retaining the former and setting the latter free. And as the atmosphere always contains carbonic acid in small quantities, the result is that plants remove carbonic acid therefrom and give out oxygen. Animals, on the other hand, have no power of living on water, carbonic acid and ammonia, nor of converting these into the complex organic substances of their bodies. That their existence may be maintained animals require to be supplied with ready-made organic compounds, and for these they are all dependent upon plants, either directly or indirectly. In requiring as food complex organic bodies, which they ultimately reduce to very simply inorganic ones, animals are thus found to differ from plants. Whilst plants are the great manufacturers in nature, animals are the great consumers. Another distinction, arising from the nature of their food, is that animals absorb oxygen and throw out carbonic acid, their reaction upon the atmosphere being exactly the reverse of that of plants. There are organisms, it must be understood, which are genuine plants so far as their nutritive processes are concerned, but which, nevertheless, are in the possession of characters which could locate them among the animals. Volvox, so abundant in our streams during the proper seasons, affords a splendid illustration of the truth of this statement. Plants, which are devoid of chlorophyll, as is the case with the Fungi, do not possess the power of decomposing carbonic acid under the influence of sunlight, but are like animals in requiring organic compounds for their food. Two points must therefore be borne in mind in regarding the general distinctions between plants and animals which we have thus briefly outlined, and these are that they cannot often be applied in practice to ambiguous microscopic organisms, and certainly not to plant-forms that are destitute of chlorophyll.