pure white which breed pure white, and some pure gray mice which breed pure gray, besides some impure forms like those of the first generation, which breed both colors. In other cases the new form if bred to the parent stock gives rise to offspring which even in the first generation are like the new form. Thus it appears that when the ancon ram was bred to an ordinary ewe the offspring were like the father. Similar cases are recorded for the merino sheep, for the japanned peacock, and for other new types. It appears here that the new type dominates in some or in all of the offspring. Darwin had in mind cases like these when he rejected the view that discontinuous variations have furnished the material for natural selection.[1]
There are, however, still other kinds of sudden and definite variations, and these, not the former, are the kind which are mainly responsible for new species. New elementary species arise not through a loss of some character of the old form (although new species may have amongst their new characters some that are due to loss), but by the appearance of a new character or characters which follow a different law of inheritance. When such elementary species are crossed with the parent type, a new type may arise, which is usually intermediate in character between the parents, but in some cases may be more like one than the other. Similar results follow when the new elementary species are crossed with each other. Thus the results are comparable to those that occur when Linnæan species are crossed, although in the case when forms belonging to two widely different Linnæan species are combined the results may be so complicated that it is beyond our power to give a satisfactory analysis of the results at present.[2]
The term mutation has been used for the sudden appearance of new variations belonging to any one of the preceding cases. Whether it may not prove necessary to use different terms for the different kinds of discontinuous variations in so far as they follow different laws of inheritance remains for the future to decide. We might, perhaps, use the term mutation (and mutant) only where new elementary species are formed; retrogression where a character is lost (elementary varieties being formed, which follow the Mendelian law of inheritance, as
- ↑ It is not clear from the records whether this class follows the Mendelian rule of inheritance, with the new form as the dominant in the first generation. From the statements that are ordinarily made, it might appear that even in the first generation some of the offspring are like one parent and some like the other, which, if true, would make these cases different from the first class given that follow Mendel's law of inheritance.
- ↑ In my book on 'Evolution and Adaptation,' I pointed out that while a single step or imitation could not he halved, yet if a number of steps were taken in the same direction the number might then be halved. This may be true but is unnecessarily hypothetical, and, as pointed out here in the text, the apparent difficulty that Darwin met with can now be explained in other ways.
