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Show 1903.] IN FANCY MICE AND RATS. 79 inter se. This point was elaborately tested. Crampe states that albinos true-bred for some generations behaved differently from extracted albinos, the former being, as he says, merely "absorbed," i. e. recessive, on crossing with colour; while extracted albinos gave, as I understand him, a mixture of ancestral forms when they were crossed with other types. This part of his paper (10. pp. 573-5) is difficult to follow ; and I cannot find any example showing precisely the nature of the distinction he means to emphasise so far as albinos are concerned. W e must here await fresh experiments. W e readily see, however, that though in respect of its albinism we may regard the albino as always the same, it may obviously be retaining other characters derived from various progenitors. Accordingly we find, as will appear, albinos apparently of the same species manifesting different properties in crossing. I suspect, however, that Crampe is here extending to the albino a generalisation really based on a mistake arising from misconception respecting the phenomenon of dominance. [See note added p. 97.] W e may now, though the evidence is imperfect, consider the significance of the appearance of these many new forms in F„. This phenomenon is a most usual result of a cross between distinct varieties. It is the source of the majority of our new garden varieties, and of many at all events of the colour-varieties of domestic animals. In general terms we can declare that the result of the cross-the "asymmetrical fertilisation," to speak strictly-is the production of a diversity of gametes. Pending histological research, we cannot tell the origin of the characters borne by these gametes; but from many circumstances it seems inevitable that they must be regarded as created in such a case partly by resolution of the character brought in by the dominant- which we therefore call a compound character, and partly by the imperfect segregation of that compound or of its components from the recessive character (and its components if it be also resoluble). In most cases the process of resolution is not complete for all the gametes; and some of the gametes are bearers of the wholly or partly unresolved character, just as all the colour-bearing gametes were in Cuenot's simpler case. The Mendelian hypothesis leads us to believe that the actual numbers of each type of gamete will be on the average definite, and that the union of any two of them will give rise to a zygote of definite character. The number of types of gametes and their several properties can only be determined on a minute analysis of each member of the series of zygotes by exhaustive breeding. No such evidence is yet complete in any one case, but we see already in certain cases that some of the F2 are homo- and some hetero-zygous, and we are beginning to suspect the ratios of the gametic forms in a few simple cases. Returning to Crampe's evidence, though the ratios are quite uncertain, we find that the several types had different properties. |
