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Show 96 MR. W. BATESON ON COLOUR-HEREDITY [May 26, Mendelian hypothesis to special cases, as with the formulation of the specific laws followed by the several characters of various animals and plants in gameto-genesis and in union by fertilisation. As in chemistry, these laws must be worked out separately case by case, and each as it is determined has the value rather of fact than of hypothesis. In dealing with this class of fact, special precautions are necessary in order to establish the identity and purity of any variety chosen for experiment. From the description of the varieties of mice given at p. 73 it will be seen that some colours may be easily confounded in description, though the pigments on which they depend have a different chemical behaviour. This is especially the case with regard to " fawn," " fawn-yellow," and " yellow." In such cases it is absolutely necessary that the presence or absence of dark pigment should be noted, and that some attempt shoidd be made to distinguish the two dark pigments from each other. In all attempts to trace laws of colour-heredity, colours of the various parts will usually have to be reckoned with. In horses, for example, the general body-colour, without that of the mane, tail, and fetlocks is likely to be an insufficient guide to the heredity. In m a n the heredity of eye-colour cannot be sufficiently investigated if it be separated from the colour of the hair, and so forth. For the prasent, therefore, Professor Pearson's conclusion that the Mendelian system does not apply to coat-colour of horses or to eye-colour in man should not be received without reserve. By neglect of the precautions named above many results may be described as conflicting with each other, which further analysis would show to be harmonious. [Note added July, 1903. When this communication was read I had not seen the important paper of Cuenot (27) dated March 1903. He states that grey mice of his F3 when crossed with albino gave several blacks. These, when bred with certain albinos produced black heterozygotes, which bred inter se gave the Mendelian 3 black to 1 albino. Some of these latter blacks were then homozygous, and from them a pure strain of blacks was raised. This strain crossed with wild musculus behaved as a simple recessive, giving grey F , with Mendelian ratios 3 grey to 1 black, or 1 grey to 1 black in F., according as Fl was mated with F,, or with black. The fact that the original black did not appear in FQ or in F calls for elucidation. It suggests a possible difference between the albinos used either in producing F, or later, some individuals having the power of resolving the grey, while others had not that power. Cuenot next records the new and important fact that the colour of the Fl produced from his black strain x albino differed according to the class of albino used. (1) Albinos extracted from |