what is conventionally termed the type; and since these unit-characters must from their behaviour in transmission be regarded as discontinuous in their nature, it follows that the variation must be discontinuous also. A present tendency of thought is to regard the discontinuous variation or mutation as the material upon which natural selection works, and to consider that the process of evolution takes place by definite steps. Darwin’s opposition to this view rested partly upon the idea that the discontinuous variation or sport would, from the rarity of its occurrence, be unable to maintain itself against the swamping effects of intercrossing with the normal form. Mendel’s work has shown that this objection is not valid, and the precision of the mode of inheritance of the discontinuous variation leads us to inquire if the small or fluctuating variation can be shown to have an equally definite physiological basis before it is admitted to play any part in the production of species. Until this has been shown it is possible to consider the discontinuous variation as the unit in all evolutionary change, and to regard the fluctuating variation as the uninherited effect of environmental accident.
The Human Aspect.—In conclusion we may briefly allude to certain practical aspects of Mendel’s discovery. Increased knowledge of heredity means increased power of control over the living thing, and as we come to understand more and more the architecture of the plant or animal we realize what can and what cannot be done towards modification or improvement. The experiments of Biffen on the cereals have demonstrated what may be done with our present knowledge in establishing new, stable and more profitable varieties of wheat and barley, and it is impossible to doubt that as this knowledge becomes more widely disseminated it will lead to considerable improvements in the methods of breeding animals and plants.
It is not, however, in the economic field, important as this may be, that Mendel’s discovery is likely to have most meaning for us: rather it is in the new light in which man will come to view himself and his fellow creatures. To-day we are almost entirely ignorant of the unit-characters that go to make the difference between one man and another. A few diseases, such as alcaptonuria and congenital cataract, a digital malformation, and probably eye colour, are as yet the only cases in which inheritance has been shown to run upon Mendelian lines. The complexity of the subject must render investigation at once difficult and slow; but the little that we know to-day offers the hope of a great extension in our knowledge at no very distant time. If this hope is borne out, if it is shown that the qualities of man, his body and his intellect, his immunities and his diseases, even his very virtues and vices, are dependent upon the ascertainable presence or absence of definite unit-characters whose mode of transmission follows fixed laws, and if also man decides that his life shall be ordered in the light of this knowledge, it is obvious that the social system will have to undergo considerable changes.
Bibliography.—In the following short list are given the titles of papers dealing with experiments directly referred to in this article. References to most of the literature will be found in (11), and a complete list to the date of publication in (3).
(1) W. Bateson, Mendel’s Principles of Heredity (Cambridge, 1902), contains translation of Mendel’s paper. (2) W. Bateson, An Address on Mendelian Heredity and its Application to Man, “Brain,” pt. cxiv. (1906). (3) W. Bateson, Mendel’s Principles of Heredity (1909). (4) R. H. Biffen, “Mendel’s Laws of Inheritance and Wheat Breedings,” Journ. Agr. Soc., vol. i. (1905) (5) W. E. Castle, “The Heredity of Sex,” Bull. Mus. Comp. Zool. (Harvard, 1903). (6) L. Cuénot, “L’Hérédité de la pigmentation chez les souris,” Arch. Zool. Exp. (1903–1904). (7) H. de Vries, Die Mutationstheorie (Leipzig, 1901–1903). (8) L. Doncaster and G. H. Raynor, “Breeding Experiments with Lepidoptera,” Proc. Zool. Soc. (London, 1906). (9) C. C. Hurst, “Experimental Studies on Heredity in Rabbits,” Journ. Linn. Soc. (1905). (10) G. J. Mendel, Versuche über Pflanzen-Hybriden, Verh. natur. f. ver. in Brünn, Bd. IV. (1865). (11) Reports to the Evolution Committee of the Royal Society, vols. i.–iii. (London, 1902–1906, experiments by W. Bateson, E. R. Saunders, R. C. Punnett, C. C. Hurst and others). (12) E. B. Wilson, “Studies in Chromosomes,” vols. i.–iii. Journ. Exp. Zool. (1905–1906). (13) T. B. Wood, “Note on the Inheritance of Horns and Face Colour in Sheep,” Journ. Agr. Soc. vol. i. (1905). (R. C. P.)
MENDELSSOHN, MOSES (1729–1786), Jewish philosopher, was born in Dessau in 1729. His father’s name was Mendel, and he was later on surnamed Mendelssohn (＝son of Mendel). He was the foremost Jewish figure of the 18th century, and to him is attributable the renaissance of the House of Israel. With this third Moses (the other two being the Biblical lawgiver and Moses Maimonides) a new era opens in the history of the Jewish people. Mendel Dessau was a poor scribe—a writer of scrolls—and his son Moses in his boyhood developed curvature of the spine. His early education was cared for by his father and by the local rabbi, David Fränkel. The latter, besides teaching him the Bible and Talmud, introduced to him the philosophy of Maimonides (q.v.). Fränkel received a call to Berlin in 1743. Not many months later a weakly lad knocked at one of the gates of Berlin. He was admitted after an altercation, and found a warm welcome at the hands of his former teacher. His life at this period was a struggle against crushing poverty, but his scholarly ambition was never relaxed. A refugee Pole, Zamosz, taught him mathematics, and a young Jewish physician was his tutor in Latin. He was, however, mainly self-taught. “He learned to spell and to philosophize at the same time” (Graetz). With his scanty earnings he bought a Latin copy of Locke’s Essay concerning the Human Understanding, and mastered it with the aid of a Latin dictionary. He then made the acquaintance of Aaron Solomon Gumperz, who taught him the elements of French and English. In 1750 he was appointed by a wealthy silk-merchant, Isaac Bernhard, as teacher to his children. Mendelssohn soon won the confidence of Bernhard, who made the young student successively his book-keeper and his partner.
Gumperz or Hess rendered a conspicuous service to Mendelssohn and to the cause of enlightenment in 1754 by introducing him to Lessing. Just as the latter afterwards makes Nathan the Wise and Saladin meet over the chess-board, so did Lessing and Mendelssohn actually come together as lovers of the game. The Berlin of the day—the day of Frederick the Great—was in a moral and intellectual ferment. Lessing was the great liberator of the German mind. He had already begun his work of toleration, for he had recently produced a drama (Die Juden, 1749), the motive of which was to prove that a Jew can be possessed of nobility of character. This notion was being generally ridiculed as untrue, when Lessing found in Mendelssohn the realization of his dream. Within a few months of the same age, the two became brothers in intellectual and artistic cameraderie. Mendelssohn owed his first introduction to the public to Lessing’s admiration. The former had written in lucid German an attack on the national neglect of native philosophers (principally Leibnitz), and lent the manuscript to Lessing. Without consulting the author, Lessing published anonymously Mendelssohn’s Philosophical Conversations (Philosophische Gespräche) in 1755. In the same year there appeared in Danzig an anonymous satire, Pope a Metaphysician (Pope ein Metaphysiker), the authorship of which soon transpired. It was the joint work of Lessing and Mendelssohn. From this time Mendelssohn’s career was one of ever-increasing brilliance. He became (1756–1759) the leading spirit of Nicolai’s important literary undertakings, the Bibliothek and the Literaturbriefe, and ran some risk (which Frederick’s good nature obviated) by somewhat freely criticizing the poems of the king of Prussia. In 1762 he married. His wife was Fromet Gugenheim, who survived him by twenty-six years. In the year following his marriage Mendelssohn won the prize offered by the Berlin Academy for an essay on the application of mathematical proofs to metaphysics, although among the competitors were Abbt and Kant. In October 1763 the king granted Mendelssohn the privilege of Protected Jew (Schutz-Jude)—which assured his right to undisturbed residence in Berlin.
As a result of his correspondence with Abbt, Mendelssohn resolved to write on the Immortality of the Soul. Materialistic views were at the time rampant and fashionable, and faith in immortality was at a low ebb. At this favourable juncture appeared the Phädon (1767). Modelled on Plato’s dialogue
