=Terminology.--=As work in the study of Mendelian inheritance has progressed and expanded the need of a more precise terminology has become evident and such is gradually being established. Thus Professor Bateson has coined the term "allelomorph" (Gk. _one another_, and _form_) to express more exactly what we have thus far been calling a pair of alternate or opposite characters. In the blue Andalusian fowls discussed, the white condition in the one parent is the allelomorph of the black condition in the other. The term generally means one of the pair of Mendelian characters themselves as expressed in the individual plants or animals but when the germinal basis of such phenomena is under discussion, it is sometimes used to refer to the determiners of such characters. And by determiner is meant simply the condition which is necessary in the germ to bring about the occurrence of a definite character. For example, when we are studying a cross between a red flower and a white flower with reference to the color factors, the difference between the two plants may lie in the fact that one produces a red coloring matter and the other does not. That is, the determiner for red is absent from the white variety. What the exact relation of color production is to the parts of the germ-cell we do not know. It could be the function of a single definite body or the resultant of several cooperating bodies. The latter is far more likely to be the case. We may suppose that a group of cooperating substances function to produce red in the red flower but that in the white flowers one of these bodies is absent or fails to perform its red-producing function. It is customary where practicable to refer to the determiner of a character by the initial letter of the name of the character. The letter when written as a capital indicates the determiner but when written as a small letter the absence of the determiner. Thus R may be taken to represent the determiner for red coloring matter and r its absence. It is convenient also to have a brief symbol to denote a given generation and for this purpose Bateson has introduced the symbol F_{1} for the hybrid progeny of the first cross, the initial letter of the word "filial." F_{2} would indicate the next generation, F_{3} the third and so on. Likewise P denotes the original parent generation. =The Theory of Presence and Absence.--=Many, if not all, allelomorphs consist of the presence and absence respectively of a given determiner. In such cases the character represented by the presence of the determiner is dominant over the character represented by the absence of a determiner. Thus in the crosses from the wild gray mice and albino mice the progeny are all gray mice since one parent had the determiner or group of determiners for grayness and the hybrid offspring must also possess it. Likewise the presence of black in black guinea-pigs is dominant to its absence in albino guinea-pigs and the resulting progeny are all black.