Seeing that art uses the signs of nature of which man is at once a product and a tool, it must in its progress follow the general course of nature. In her development of life, nature is chiefly concerned in the improvements of types for her own purposes, and only uses the individual in so far as he can assist in this end, while the natural instinct of the individual is to conserve and improve his type. The art which represents life is compelled to deal chiefly with types, for it is only by the use of a type that the artist can apply his imagination to the production of high beauty, to whatever extent he may use the individual to help him in this purpose, and it is instinctive in the human being to maintain and improve the æsthetic attraction of the species. The highest art, as the highest work of nature, consists of the presentation of a perfected type. The artist therefore must consider the species before the individual; the essential before the accidental; the general before the particular. The living signs of nature with which art deals are of two classes. In the one sign the position of parts is the same throughout the species, and is fixed and invariable, as in fully developed animals; in the other the position is irregular, and variable within limits, as in plants. In the latter case the position of parts may be commonly varied indefinitely without a sense of incongruity arising, as in a tree, and hence there can be no conceivable general form or type upon which art may build up perfected parts and proportions. In respect of such a sign therefore, art cannot improve, or appear to improve, upon nature, by combining perfected parts into a more beautiful whole than nature provides.