The Book of Household Management by Mrs. Beeton

History of the Vegetable Kingdom, "may be aptly compared to the primary colours of the prismatic spectrum, which are so gradually and intimately blended, that we fail to discover where the one terminates and where the other begins. If we had to deal with yellow and blue only, the eye would easily distinguish the one from the other; but when the two are blended, and form green, we cannot tell where the blue ends and the yellow begins. And so it is in the animal and vegetable kingdoms. If our powers of observation were limited to the highest orders of animals and plants, if there were only mammals, birds, reptiles, fishes, and insects in the one, and trees, shrubs, and herbs in the other, we should then be able with facility to define the bounds of the two kingdoms; but as we descend the scale of each, and arrive at the lowest forms of animals and plants, we there meet with bodies of the simplest structure, sometimes a mere cell, whose organization, modes of development and reproduction, are so anomalous, and partake so much of the character of both, that we cannot distinguish whether they are plants or whether they are animals."