Page:Popular Science Monthly Volume 85.djvu/324

320 as for example, in the skin color of mulattoes; such cases were called by Galton "blending" inheritance (Fig. 47). Sometimes characters appear in offspring which were "latent" in the parents but were "patent" in one or more of the grandparents; such skipping of a generation, during which a character remains "latent," has long been known as "atavism." At other times characters which were present in distant ancestors, but which have since dropped out of sight, or have remained "latent," reappear in descendants; such cases are known as "reversions."

In still other cases certain characters appear only in the male sex, others only in the female, this being called "sex-limited" inheritance; while in some instances characters are transmitted from fathers through daughters to grandsons or from mothers to sons, all such cases being known as "sex-linked" inheritance.

2. New Characters or Mutations.—But in addition to these permutations in the distribution and combination of ancestral characters new and unexpected characters sometimes develop in the offspring, which were not present, so far as shown, in any of the ascendants, but which, after they have once appeared, are passed on by heredity to descendants. Such inherited variations are usually of two kinds, continuous or slight, and discontinuous or sudden variations. The latter are especially noticeable when variations occur in the normal number of parts, as in four-leaved clover, or six-fingered men, and such numerical variations have been called by Bateson "meristic." However sudden variations may include any marked departure from the normal type, in color, shape, size, chemical compositions, etc. Such sudden variations have long been known to breeders as "sports," and both Darwin and Galton pointed out the fact that such sports have sometimes given rise to new races or breeds, though Darwin was not inclined to assign much importance to them in the general process of evolution. Galton, on the other hand, maintained that variations, or what would now be called "continuous variations," can not be of much significance in the process of evolution, but that the case is quite different with "sports."

More recently the entire biological world has been greatly influenced by the "mutation theory" of de Vries, which has placed a new emphasis upon the importance of sudden variations in the process of evolution. At first de Vries was inclined to emphasize the degree of difference