That inheritance from both parents is only a secondary consideration in regard to the increase of a part by artificial selection is made evident by the fact that _many secondary sexual characters_ have been modified, although the breeder selected only in regard to one parent. Nevertheless in this very domain the greatest results have been achieved; witness the Japanese breed of cocks with tail-feathers six feet long. This astonishing result has been reached by the strictest selection of the cocks in which the feathers were a little longer than those of other cocks, and the increase in the length of feathers depended--according to our theory--simply on the fact that, by the selection of the determinants which were already varying in the direction of increased length, this process of increase was guarded from interruption by chance unfavourable conditions of nutrition. The continuance of variation in the upward direction in which it had already started is not effected directly by personal selection, but is so indirectly, for without this constant fresh intervention of selection the increase would be apt to come to a standstill, or the variation might even take a contrary direction. There are two other factors operative to which we have not yet given sufficient attention. They are, the multiplicity of the ids in every germ-plasm, and sexual reproduction. If--as we must assume--each germ-plasm is made up of several or many ids, there must be several or many determinants of each part of the organism, for each id contains potentially the whole organism, though with some individuality of expression. The child is thus not determined by the determinants of a single id, but by those of many ids, and the variations of any part of the body do not depend on the variations of a single determinant _X_, but on the co-operation of all the determinants _X_ which are contained in the collective ids of the relevant germ-plasm. Thus it is only when a majority of the determinants have varied upwards or downwards that they dominate collectively the development of the part _X´_ and cause it to be larger or smaller.