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Show 1903.] IN FANCY MICE AND RATS. 95 greater than the amount of the same pigment in the original compound colour. But this consideration cannot be allowed much weight, seeing that there may be an excess of pigment in heterozygotes produced even from two gametes apparently bearing no pigment elements at all {cf. p. 76). In the chemistry of pigmentation there may perhaps be interactions and cancellings so complex as to make this particular problem as yet quite insoluble. Fuller analysis is especially needed also to determine the place of the pied and diluted colour-bearing gametes in the series, but it is fairly certain that they must be regarded as due to disintegration and imperfection of resolution of the colour from the albino character. Future experiment must decide the conditions determining resolution. Cuenot, as I understand his paper, got none in the main experiment with wild mice; but he states that he obtained yellows, blacks, and pieds " accessoirement" (perhaps by introducing some coloured fancy strain ?). From this survey of evidence mostly already published, it is clear that Mendelian analysis provides a means of elucidating a large part of the phenomena. The majority of the observations are in accord with the Mendelian hypothesis in a simple form. The true solution of several subordinate problems still remains obscure. The value of the Mendelian analysis will be the more appreciated when it is remembered that previously the whole body of facts must have been regarded as a hopeless entanglement of contradictions, as reference to any non-Mendelian discussion even of these very phenomena will show. As I have elsewhere pointed out, the central phenomenon in Mendelian heredity is segregation. The characters in simplest cases are treated as units in gameto-genesis. In more complex cases there is resolution, sometimes also disintegration and imperfect segregation, leading to the formation of fresh units. The gametes bearing these units are produced in numerical proportions which on an average are also definite, but as yet these proportions have only been determined in the simple cases. There is no doubt that further experiment will determine them in complex cases also. It is the object of Mendelian analysis to determine (1) the constitution of the several types of gamete produced by each type of zygote ; (2) the numerical proportions in which each type of gamete is produced ; (3) the specific result of the union of any two of the types of gamete in fertilisation. Though for convenience we may still speak of inheritance as being " Mendelian " or " non-Mendelian," we are rapidly passing-out of the initial phase of the inquiry in which such expressions are demanded. In our further investigations we are concerned not so much with the question of the applicability of the simplest |
