in such a manner, and one which has developed from an egg fertilized by the male is that it is made up of characteristics from only one source, the maternal. =Dual Ancestry an Aid in Studying Heredity.--=Although we have the factors of heredity in a more simplified form in the case of asexual transmission, as a matter of fact most of our insight into the problems of heredity has been attained from a study of sexually reproducing forms, because the very existence of two sets of more or less parallel features offers a kind of checking up system by which we can follow a given characteristic. =Reversion.--=Occasionally, however, plants and animals do not develop the complete individuality we might expect, but stop short at or re-attain some ancestral stage along the line of descent, and thus come to resemble some progenitor perhaps many generations back of their own time. Thus it is well known that as regards one or more characteristics a child may resemble a grandparent or often some remote ancestor much more closely than it does its immediate parent. The reappearance of such ancestral traits the student of heredity designates as _Reversion_ or _Atavism_. Reversion may occur apparently in any class of plants or animals. It is especially pronounced among domesticated forms, which through man's selection have been produced under more or less artificial conditions. For example, among fancy breeds of pigeons, there may be an occasional return to the old slaty blue color of the ancestral rock-pigeon, with two dark cross-bars on the wings, from which all modern breeds have been derived. This is almost sure to happen if the fancy varieties are inter-crossed for two or three generations. Another example of reversion frequently cited is the occasional reappearance in domestic poultry of the reddish or brownish color pattern of the ancestral jungle-fowl to which, among modern forms, the Indian game seems most nearly related in color. Still another example is the cross-bars or stripes occasionally to be seen on the forelegs of colts, particularly mules, reminiscent of the extinct wild progenitors which were supposedly striped. Fig. 1, p. 9, is a picture of a hybrid between the common fowl and the guinea-fowl. The chevron-like markings on certain feathers show a reversion to a type of color pattern that is prevalent among both the primitive pheasants (the domestic chicken is a pheasant) and the primitive guinea-fowls. Although the common spotted guinea-fowl may be crossed with a black chicken which shows no trace of barring, nevertheless the hybrid offspring are likely to bear a chevron-like pattern such as that shown in the picture.