Continuous and Discontinuous Variations and Their Inheritance in Peromyscus. III

Abstract

1. In the preceding pages, the differences, structural and pigmental, which distinguish four geographical races of deer-mice are discussed in some detail. The pigmental differences relate to a considerable range of more or less independently varying characters, affecting both the intensity and the extensity of the pigment in the hair and skin. They are found to be, in a general way, correlated with certain elements of the physical environment, while the structural differences do not appear to be so correlated. 2. All of these differences, structural and pigmental, are found to be differences of degree, revealed through a comparison of mean or modal conditions rather than of individual animals. In comparing the less divergent of these races with one another, the frequency polygons, for any given character overlap broadly. 3. These subspecific differences, and even the minor differences which distinguish one narrowly localized subrace from the parent form, are found to be hereditary, as evidenced by their persistence when environmental conditions are interchanged. 4. The gradations, in certain of these characters by which individuals of the same race differ from one another are found to be strongly hereditary. 5. Hybrids between even the most divergent of these four races are predominantly intermediate in character, both in the F1 and the F2 generations. In both of these generations a wide range of variability is exhibited, which, however, is little if any greater in the F2 than in the F1. 6. In contrast to the sensibly continuous variation and sensibly blended inheritance shown in respect to these subspecific characters, is the behavior of certain "mutations.'' Here we meet with typical discontinuous variation, and inheritance of the strictly alternative or Mendelian type. It is insisted that the burden of proof rests upon those, who contend that these two types of variation and inheritance are reducible to a single category, that of discontinuity. Anything like a proof of this contention appears to be thus far lacking.