While it must be conceded that Galen, following Aristotle, had never seriously questioned the fixity of species, the following quotation from his work _On Habits_ (chap. ii.) will show that he must have at least had occasional glimmerings of our modern point of view on the matter. Referring to _assimilation_, he says: "Just as everything we eat or drink becomes _altered in quality_, so of course also does the altering factor itself become altered.... A clear proof of the assimilation of things which are being nourished to that which is nourishing them is the change which occurs in plants and seeds; this often goes so far that what is highly noxious in one soil becomes, when transplanted into another soil, not merely harmless, but actually useful. This has been largely put to the test by those who compose memoirs on farming and on plants, as also by zoological authors who have written on the changes which occur according to the countries in which animals live. Since, therefore, not only is the nourishment altered by the creature nourished, but the latter itself also undergoes some slight alteration, _this slight alteration must necessarily become considerable in the course of time_, and thus properties resulting from prolonged habit must come to be on a par with natural properties." Galen fails to see the possibility that the "natural" properties themselves originated in this way, as activities which gradually became habitual--that is to say, that the effects of _nurture_ may become a "second nature," and so eventually _nature_ itself. The whole passage, however, may be commended to modern biologists--particularly, might one say, to those bacteriologists who have not yet realised how extraordinarily _relative_ is the term "specificity" when applied to the subject-matter of their science. [4] In terms of filtration, diffusion, and osmosis. BIBLIOGRAPHY Codices Bibliothèque Nationale. Paris. No. 2267. Library of St. Mark. Venice. No. 275. Translations Arabic translations by Honain in the Escurial Library, and in the Library at Leyden. Hebrew translation in the Library at Bonn. Latin translations in the Library of Gonville and Caius College (MSS.), No. 947; also by Linacre in editions published, London, 1523; Paris, 1528; Leyden, 1540, 1548, and 1550; also by C.G. Kühn, Leipzig, 1821. Commentaries and Appreciations