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In 1902 Professor Bateson published, under the title Mendel's Principles of Heredity: a Defence, a translation of Mendel's paper on Hybridization, together with a brief summary of Mendelism as then developed. The book quickly went out of print, and was not republished. The object of the present work, which in a sense takes the place of its predecessor, is to give a succinct account of discoveries in regard to heredity made by the application of Mendel's method of research. Theoretical considerations, the bearing of the new facts on the great problems of biology, are here reduced to the briefest possible indication; they will be dealt with separately in another book, based on the author's Silliman Lectures of 1907.

We have before us, then, a sort of text-book of Mendelism, in so far as the doctrine is capable of representation in text-book form, written by one of its foremost expositors. Ch. i, Introductory: Mendel's Discovery, gives a brief historical outline, and thereafter illustrates and defines various technical terms: dominant and recessive, segregation and allelomorphism, homozygote and heterozygote, purity of type. "The fact of segregation was the essential discovery which Mendel made. . . . The dominance of certain characters is often an important but never an essential feature of Mendelian heredity. . . . Purity of type is dependent on gametic segregation, and has nothing to do with a prolonged course of selection, natural or artificial." The account given of these fundamental matters is condensed, and needs careful reading, but is sufficiently clear to the advanced student.

Ch. ii, The Material Investigated, begins with a very useful selected list of structural characters in plants (36) and animals (25) whose inheritance follows the general rules described in the preceding chapter. References to the original memoirs, and brief summaries, are appended in every case. The author then turns to the subject of color, which is presently to occupy him through several chapters, and enumerates the animals and plants in which color characters have been shown to have a Mendelian inheritance. In conclusion, he touches on various general questions. No class of characters has as yet been identified to which the Mendelian system is demonstrably inapplicable, though the future cannot, of course, be foretold. One meristic character (brachydactyly in man) is Mendelian; the study of such characters is now of especial importance. Mendelian principles have been proved to apply to wild types, and are thus not confined to unions of pure races. There is no distinction between inter-racial and intra-racial heredity. Dominance is not necessarily an attribute of the phylogenetically older character. A dominant character is the condition due to the presence of a definite factor; the corresponding recessive owes its condition to the absence of the same factor. The chapter ends with a statement of the salient differences between the Mendelian and the Galtonian theories of inheritance. The statement is admirably clear; and, indeed, the sharp division drawn between Mendelism on the one hand and biometry on the other is one of the best features of the book.