If the germ-plasm consists of ids, these ids collectively must determine the structure, the whole individuality--let us say, briefly, the 'type' of the offspring; it is the resultant of all the different impelling forces which are contained in the different ids. If these were all equally strong, and all operating in the same direction, they would necessarily all have the same share in the resultant of development, the 'type' of the child. But this is not the case. Numerous experiments on the hybridization of two species of plant have taught us that the descendants of such hybridization usually maintain a medium between the ancestral species; but it is not always the case, for in many hybrids the character of one species, whether paternal or maternal, preponderates in the young plant. We recognize the same thing still more clearly in Man, whose children by no means always maintain a medium between the characters of the two parents, but frequently resemble one--the father or the mother--much more strongly than the other. How can this fact be theoretically explained? Must we ascribe to the ids of the father or of the mother a greater determining power? Without excluding such an assumption as on _a priori_ grounds inadmissible, I am inclined to believe that we do not require it to explain _this_ phenomenon. For, if we take our stand simply on the fact of the preponderance of one parent, it follows directly from this that not all the ids control the type of the child, let the cause of the non-co-operation of some of them be what it may. But if in this case only a portion of the ids contained in the germ-plasm controls the type, this combination of ids suffices to make the child resemble one parent, the father, for instance, and consequently half the number of ids is sufficient in some circumstances to determine the child--taking for granted that the one-sidedness of the inheritance is complete, which never actually happens. But the half number of ids can only suffice if it includes the same combinations of ids which have determined the type in the case of the father; as soon as one or more ids of this particular combination are replaced by others the paternal germ-plasm alone is not enough to call forth complete resemblance in the child. But, at the reduction, a change of arrangement of ids takes place, and a new combination arises, and thus each germ-cell receives its particular group of ids. It may thus happen that, in one particular sperm-cell, exactly the same group of ids is contained as that which determined the type of the father, and that the same is true of a particular egg-cell in regard to the type of the mother. Let us now assume that a sperm-cell and an egg-cell meet, which contain both those groups of ids which had determined the type of the father and of the mother; if the determining power of the maternal and paternal ids were equal a child would result which would maintain the medium between father and mother.