As a corollary to this interpretation of the Aristotelian doctrine of ideal imitation, there appeared a tendency to ignore more and more the element of personal feeling in poetry,[18] and to concentrate attention on the formal elements of the art. This tendency, reinforced by the authority of the Horatian tag, _ut pictura poesis_ (“as is painting, so is poetry”), led naturally, and in an ever-increasing degree, to the formal identification of poetry with painting. Critics became accustomed to discussing the elements in the art of writing that correspond to the other elements in pictorial art, such as light, colour, expression, etc. And as the poet was to be an imitator of accepted models, so also he was to be imitative and traditional in using poetical colouring, in which phrase were included, as Dryden wrote, “the words, the expressions, the tropes and figures, the versification, and all the other elegancies of sound.”[19] That this parallelism directly encourages the growth of a set “poetic diction” is obvious; the poet’s language was not to be a reflection of a genuine emotion felt in the mind for his words, phrases, and figures of speech, his _operum colores_,[20] he must not look to Nature but to models. In brief, a poetical _gradus_, compiled from accepted models, was to be the ideal source on which the poet was to draw for his medium of expression. It is not necessary to dwell long on this pseudo-classical confusion of the two arts, as revealed in the critical writings of Western Europe down to the very outbreak of the Romantic revolt.[21] In English criticism, Dryden’s “Parallel” was only one of many. Of the eighteenth century English critics who developed a detailed parallelism between pictorial and plastic art on the one hand and poetry on the other, maintaining that their standards were interchangeable, the most important perhaps is Spence, whose “Polymetis” appeared in 1747, and who sums the general position of his fellow-critics on this point in the remark, “Scarce anything can be good in a poetical description which would appear absurd if represented in a statue or picture.”[22] The ultimate outcome of this confusion of poetry and painting found its expression in the last decade of the eighteenth century in the theory and practice of Erasmus Darwin, whose work, “The Botanic Garden,” consisted of a “second part,” “The Loves of the Plants,” published in 1789, two years before its inclusion with the “first part” the “Economy of Vegetation,” in one volume. Darwin’s theory of poetry is contained in the “Interludes” between the cantos of his poems, which take the form of dialogues between the “Poet” and a “Bookseller.” In the Interlude to Canto 1 of Part II (“The Loves of the Plants”) he maintains the thesis that poetry is a process of painting to the eye, and in the cantos themselves he proceeds with great zeal to show in practice how words and images should be laid on like pigments from the outside.