Ch. iii, Numerical Consequences and Recombinations, deals illustratively (combs of fowls, heterostylism in Primula) with the proportions of the F generation and with novel types produced by recombinations.
The succeeding five chapters discuss in detail the problem of heredity of color. "Taking the evidence respecting the genetics of color as a whole, . . . there can be no reasonable doubt that with rare exceptions it will be found possible to express the whole series of phenomena as due to the combination and recombination of a limited number of recognizable factors, which are treated by the cell-divisions of gametogenesis as units. . . . One positive deduction cannot be overlooked: that the organism is so built that definite additions to, or subtractions from, its totality may readily be made by Variation, and that the consequence of such alteration of the ingredients may be recognizably definite or, to use another term, specific."
Ch. ix deals with gametic coupling and spurious allelomorphism. Certain phenomena "indicate a system of segregation taking place in such a way that gametes presenting certain . . . combinations occur with greater frequency than the others." Instances are drawn from the pollen-shape, contabescence of anthers, and color of the Sweet Pea. Spurious allelomorphism occurs when factors concerned with features of organization which seem to have no special physiological association behave as allelomorphic to each other. "Two dominant or 'present' factors behave as if in the cell-divisions of gametogenesis they repelled each other. . . . The dividing cell being AaBb, the daughter-cells are respectively Ab and aB." The author recognizes the possibility of disturbance by selective attraction between different kinds of gametes (selective mating), though much more evidence is required for anything like demonstration.
Ch. x reviews the facts relating to Heredity and Sex, with strict limitation, of course, to Mendelian experiments. The main outcome is that in certain forms the female is a sex-heterozygote, with femaleness dominant; the female is a hybrid, female-male, while the male is pure male, or male-male. Since, on the other hand, the cytologists show that in most orders of Insects proof that the male is heterozygous can be obtained, the author concludes: "Improbable as at first sight it may appear, the view that most commends itself to me is that in different types Sex may be differently constituted." The fact that the females of a true-breeding strain may be hybrid in some important respect, while the males are not, brings us a step nearer the discovery of the nature of Variation.
Ch. xi, on Double Flowers, we may pass over. Ch. xii, on Evidence as to Mendelian Inheritance in Man, brings together such observations of inherited traits, normal and abnormal, as can be found; the evidence, however, is scanty, since man has for some reason or other hardly, one would think, for the reason alleged by the writer, the "special difficulties attending the study of human heredity;" since these are as great for Galtonians as for Mendelians been left almost entirely to the biometrists. Eye-color, brachydactyly, cataract, tylosis, etc., are discussed in sufficient detail, and the chapter ends with a note on collecting evidence as to human descent: a note which gains in interest by the insertion of a slip, on which Professor Bateson confesses that his representation of the inheritance of color-blindness "contains a serious error." The reviewer may take occasion to say that, not only here but throughout the work, the author makes an impression of the utmost candor and frankness; mistakes and ignorance are signalized, no less than success and successful prediction (cf. p. 209, the mulatto; p. 128, the Basset hound).
