The origin and development of plant-life, or, as it is termed, evolution, has made rapid advancement as a study during the century. That there has been an adaptation to conditions in some respects, as contended by Mr. Darwin and his followers, must be correct. The oak and other species of trees must have been formed before mistletoe and other parasites could grow on them. In the common Dodder—species of _Cuscuta_—the seeds germinate in the ground like ordinary plants. As soon as they find something to attach themselves to, they cut loose from mother earth and live wholly on the host. As a speculation it seems plausible that all parasites have arisen in this way. Some, like the mistletoe, having the power, at length, to have their seeds germinate on the host-plant, have left their terrestrial origin in the past uncertain. A number of parasites, however, do not seem to live wholly on the plants they attach themselves to. These are usually destitute of green color. The Indian pipe, snow plant of the Pacific Coast, and Squaw root of the Eastern States are examples; the former called ghost-flower from its paleness. These plants have little carbonaceous matter in their structure, and hence are regarded as having formed a kind of partnership with fungi. This is known now as symbiosis, or living together of dissimilar organisms, each dependent mutually. The fungus and the flowering plant in these cases are necessary to the existence of each other. They demand nitrogen instead of carbonhydroids. The Squaw root, _Conopholis Americana_, though attached to the subterranean portions of the trunks of trees, is probably sustained by the fungus material in the old bark, or even in the wood, rather than by the ordinary food of flowering plants. Lichens, as it is now well known, are a compound of fungi and water weeds (algæ), and this doctrine of symbiosis is regarded as one of the great advances of the century. It is but fair to say that the doctrine of evolution by the influence of external conditions in the change of form, though widely accepted at this time, is not without strong opponents, who point to the occasional development or suppression of parts on the same plant, though the external conditions must be the same. For instance, there are flowers that have all their parts regular, as in the petals of a buttercup; and irregular, as in the snap-dragon or fox-glove. But it has been noted that irregular flowers have pendulous stalks, while the regular ones are usually erect. But once in a while, on the same plant, flowers normally drooping will become erect. In these cases the flowers are regular. In the wild snap-dragon or yellow toad-flax, _Linaria vulgaris_, one of the petals is developed into a long spur; the other four petals have, in early life, become connate and transformed into parts of the flower wholly unlike ordinary petals. But now and then the original petals will all develop spurs, resulting in the condition technically known as peloria.