This hybrid remains true to the description given. In some years I cultivated two generations [260] so as to be able to compare them with one another, but did not find any difference. The most interesting point however, is the likeness between the first generation, which obviously must combine in its internal structure the units of both parents, and the second and later generations which are only of a derivative nature. Next to this stands the fact that in each generation all individuals are alike. No reversion to the parental forms either in the whole type or in the single characteristics has ever been observed, though the leaves of some hundreds, and the spikes and flowers of some 150 individual plants have been carefully examined. No segregation or splitting up takes place. Here we have a clear, undoubted and relatively simple, case of a true and pure species hybrid. No occurrence of possible varietal characteristics obscures the result, and in this respect this hybrid stands out much more clearly than all those between garden-plants, where varietal marks nearly always play a most important part. From the breeder's point of view our hybrid _Oenothera_ would be a distinct gain, were it not for the difficulty of its propagation. But to enlarge the range of the varieties this simple and stable form would need to be treated anew, by [261] crossing it with the parent-types. Such experiments however, have miscarried owing to the too stable nature of the unit-characters. This stability and this absence of the splitting shown by varietal marks in the offspring of hybrids is one of the best proofs of unisexual unions. It is often obscured by the accompanying varietal marks, or overlooked for this reason. Only in rare cases it is to be met with in a pure state and some examples are given of this below. Before doing so, I must call your attention to another feature of the unbalanced unions. This is the diminution of the fertility, a phenomenon universally known as occurring in hybridizations. It has two phases. First, the diminished chance of the crosses themselves of giving full crops of seed, as compared with the pure fertilization of either parent. And, secondly, the fertility of the hybrids themselves. Seemingly, all grades of diminished fertility occur and the oldest authors on hybrids have pointed out that a very definite relation exists between the differences of the parents and the degree of sterility, both of the cross and of the hybrid offspring. In a broad sense these two factors are proportionate to each other, the sterility being the greater, the lesser the affinity between the parents. Many writers have [262] tried to trace this rule in the single cases, but have met with nearly unsurmountable difficulties, owing chiefly to our ignorance of the units which form the differences between the parents in the observed cases.