Page:Popular Science Monthly Volume 85.djvu/443

Rh length remained intermediate between that of the parents. He found the same thing true of length and breadth of the skull (Fig. 65) and of the size of other portions of the skeleton, and he concluded that such quantitative characters are not inherited in Mendelian fashion.

Quite recently MacDowelL working on the inheritance of size in rabbits, concludes that this, as well as other quantitative differences between parents which appear to blend in the offspring, such as Castle's case of ear length in rabbits, is due not to a single factor, as in the case of Mendel's tall and dwarf peas, but to several factors. Consequently, in the formation of the germ cells there is not a clear segregation of all the factors for tallness, or large size or long ears, in half the germ cells, and their total absence in the other half of those cells, but some of these factors go into certain cells and others into others, as in the case of dihybrids, trihybrids or polyhybrids. As a result offspring appear more or less intermediate in size between their parents.

Thus it is possible to explain even "blending" inheritance as due not to the real fusion or blending of inheritance factors, but to varying combinations of numerous or multiple factors, according to the Mendelian rules. The Mendelian principle of segregation has been found to be of such general occurrence that there is a strong inclination among Mendelians of the stricter sort to make it universal, and to explain all cases of blending inheritance as due to incomplete dominance and to multiple factors. Whether or not such attempts may prove completely successful it is still too soon to say.

The study of human inheritance must always be less satisfactory and conclusions less secure than in the study of lower animals for the following reasons: In the first place there are no "pure lines," but the most complicated intermixture of different lines. In the second place experiments are out of the question and one must rely upon observation and statistics. There have been less than 60 generations of men since the beginning of the Christian era, whereas Jennings gets as many generations of Paramecium within two months and Morgan almost as many generations of Drosophila within two years. Finally the number of offspring are so few in man that it is difficult to determine what all the hereditary possibilities of a family may he. Bearing in mind these serious handicaps to an exact study of inheritance, it is not surprising that the method of inheritance of many human characters is still uncertain.

Davenport and Plate have catalogued more than sixty human traits which seem to be inherited in Mendelian fashion. About fifty of these represent pathological or teratological conditions, while only a relatively small number are normal characters. This does not signify that the