In order to distinguish these two great types of fertilization we will use the term bisexual for the one and unisexual for the other. The term balanced crosses then conveys the idea of complete bisexuality, all unit-characters combining in pairs. Unbalanced crosses are those in which one or more units do not find their mates and therefore remain unpaired. This distinction was proposed by Macfarlane when studying the minute structure of plant-hybrids in comparison with that of their parents (1892). [256] In the first place it shows that a species hybrid may inherit the distinguishing marks of both parents. In this way it may become intermediate between them, having some characters in common with the pollen-parent and others with the pistil-parent. As far as these characters do not interfere with each other, they may be fully developed side by side, and in the main this is the way in which hybrid characters are evolved. But in most cases our existing knowledge of the units is far too slender to give a complete analysis, even of these distinguishing marks alone. We recognize the parental marks more or less clearly, but are not prepared for exact delimitations. Leaving these theoretical considerations, we will pass to the description of some illustrative examples. In the first place I will describe a hybrid between two species of _Oenothera_, which I made some years ago. The parents were the common evening-primrose or _Oenothera biennis_ and of its small-flowered congener, _Oenothera muricata_. These two forms were distinguished by Linnaeus as different species, but have been considered by subsequent writers as elementary species or so-called systematic varieties of one species designated with the name of the presumably older type, the _O. biennis_. Varietal differences in a physiologic sense they [257] do not possess, and for this reason afford a pure instance of unbalanced union, though differing in more than one point. I have made reciprocal crosses, taking at one time the small-flowered and at the other the common species as pistillate parent. These crosses do not lead to the same hybrid as is ordinarily observed in analogous cases; quite on the contrary, the two types are different in most features, both resembling the pollen-parent far more than the pistil-parent. The same curious result was reached in sundry other reciprocal crosses between species of this genus. But I will limit myself here to one of the two hybrids. In the summer of 1895 I castrated some flowers of _O. muricata_, and pollinated them with _O. biennis_, surrounding the flowers with paper bags so as to exclude the visits of insects. I sowed the seeds in 1896 and the hybrids were biennial and flowered abundantly the next year and were artificially fertilized with their own pollen, but gave only a very small harvest. Many capsules failed, and the remaining contained only some few ripe seeds.