Science tends more and more to show that a closer affinity exists between plants and animals than was formerly believed, and consequently the old "hard and fast" division made by older naturalists cannot now, in the presence of the facts established by microscopic research, be maintained. Some animals, as, for example, the sea-anemone, have no power of locomotion or the ability to effect changes of place at will; on the other hand, some plants are endowed with the power of voluntary movements, apparently spontaneous and independent. In certain cases these movements are effected by means of little vibrating hairs or cilia, in others, as the Diatomaceæ, and Desmidiæ, they are not produced by cilia, but by some other means. In general terms the differences between animals and plants may be stated as follows, it being borne in mind that the rules are not universally applicable, some fungi, for instance, cannot live on inorganic substances alone, while some of the lower forms of animal life act like plants and manufacture organic compounds out of inorganic materials.
1. Plants live on purely inorganic substances, such as water, carbonic acid and ammonia, and they have the power of making out of these true organic substances, such as starch, cellulose, sugar, etc. Plants, therefore, take as food very simple bodies, and manufacture them into more complex substances, so that plants are the great producers in nature.
2. Plants in the process of digestion break up carbonic acid into the two elements of which it is composed, namely, carbon and oxygen, keeping the carbon and setting free the oxygen. As carbonic acid occurs always in the air in small quantities, the result of this is that plants remove carbonic acid from the atmosphere, and give out oxygen.
3. Animals, on the other hand, have no power of living on inorganic matters, such as water, carbonic acid, and ammonia. They have no power of converting these into the complex organic substances of which their bodies are composed. On the contrary, animals require to be supplied with ready-made organic compounds if their existence is to be maintained. These they can only get in the first place from plants, and therefore are all dependent upon plants for food either directly or indirectly. Animals, therefore, differ from plants in requiring as food complex organic bodies, which they ultimately reduce to very much simpler inorganic bodies. Whilst plants, then, are the great manufacturers in Nature, animals are the great consumers. Another distinction arising from the nature of their food is that whilst plants decompose carbonic acid, keeping the carbon and setting free the oxygen, animals absorb oxygen and give out carbonic acid, so that their reaction upon the atmosphere is the reverse of that of plants.
A certain analogy may be observed between the organs of life in plants and those of animals. If, for example, we take a thin transverse slice of the stem of any plant, and cut across that stem and immerse it in pure water, and place it under a microscope, we shall find that it
