Page:Popular Science Monthly Volume 67.djvu/68

62 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.

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.

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