The first principles of heredity; with 75 illustrations and diagrms . ach setof determinants trying to impress during the developmentof the organism a widely different constitution on thesuccessive organic parts. While the homologous determinants are identical intheir effect as regards the part of the organism which theydetermine, they may differ in allied species or varietieswith regard to the quality they impress on that same part. * Much of the remainder of this chapter is very compUcated,but has to be given for the sake of completeness. The beginnershould not despair, but may, if unable to
The first principles of heredity; with 75 illustrations and diagrms . ach setof determinants trying to impress during the developmentof the organism a widely different constitution on thesuccessive organic parts. While the homologous determinants are identical intheir effect as regards the part of the organism which theydetermine, they may differ in allied species or varietieswith regard to the quality they impress on that same part. * Much of the remainder of this chapter is very compUcated,but has to be given for the sake of completeness. The beginnershould not despair, but may, if unable to understand it, leave itout, and reserve it for later perusal, when he has mastered thesubject better. THEORIES OF HEREDITY 71 For instance, in two related varieties of butterflies a certainspot on the wing may be in the one variety of a browncolour, in the other red. The determinants, though homol-ogous in both cases, will;tend to impart a different colourto that special spot on the wing : they will be, as Weismannhas expressed it, heterodynamous in their action ;. Fig. 40.—Blended Inheritance in IvEAVes of Willow. {From J. A. Thomson, Heredity.) A and C, the two parents ; B, the hybrid offspring. while determinants impressing the same quality on anygiven part of the organism would be homodynamous in their effect. Now, as the development of any cell during ontogenesisis determined by only one kind of determinants of thegerm-plasm, and as in amphimixis homologous deter-minants of the father and mother organism unite, it is the 1 72 THE FIRST PRINCIPLES OF HEREDITY combined influence of these homologous determinantswhich finally determines the character of any given partof the offspring. We can now, after the above explanation, understandhow it comes about that, in the crossing of two closelyrelated varieties, the resulting hybrid shows characteristicsintermediate between those of the parents. The two crossedvarieties possess an equal number of determinants, whichare all homol
Size: 1474px × 1695px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublisherlondo, bookyear1910
