On Twin Hybrids
In the group Onagra of the evening primroses the hybrids between different species are, as a rule, constant and uniform through the succeeding generations. In this respect they comply with the common rule for the majority of the characters of specific hybrids. Only, on account of the scarcity of regressive marks in this group, the phenomenon of constancy is allowed to show itself pure and complete. An exception is afforded by Oenothera brevistylis, the character of which splits up according to
... ts up according to the formulae of MENDEL. ' As an example of a constant hybrid race I quote the 0. muricataX biennis. It has been found uniform through at least four generations. In the second, the one in which Mendelian marks are seen to split, I cultivated over 8o flowering plants and over ioo rosettes, but no differences could be detected. Last summer (I907) I cultivated, on neighboring beds, 35 plants of the first and i8 of the fourth generation. Both groups produced a number of flowering stems, but, apart from the ordinary fluctuating variability, the characters were exactly the same in all the specimens. They had the flowers almost like those of 0. biennis, but dense and richly flowered spikes like the 0. muricata. The types of the species used for this experiment were the forms which occur everywhere on waste places throughout Europe, having been introduced, the first from Virginia and the second from Canada, about three centuries and one century ago respectively. They are probably the types on which LINNAEUS based his descriptions of the species. In the United States, however, these Linnean species con-I Die Mutationstheorie 2:429. 40I sist of quite a number of subspecies, of which I collected over a dozen in Kansas, Missouri, Illinois, and elsewhere, during the summer of I904. On sowing them in my experiment garden, I observed them to be fairly distinct, each constituting a sharply defined type. The form which is the most common throughout the United States is not the same as the one introduced into Europe, neither for 0. biennis nor for 0. muricata, as has also been pointed out by MAcDOUGAL for 0. biennis. All these numerous elementary species agree with one another in a most interesting character. Their anthers touch the stigma, open themselves in the bud, and produce fertilization before the flower opens. The process is almost entirely finished before the insects are admitted. Crosses seem to be very rare in nature, although they do occur, since I collected the hybrid of 0. muricata and 0. biennis in the dunes near Amsterdam, and even more than once. A simple means of pure self-fertilization may be derived from this pollination within the bud. I cut the buds of our 0. biennis one or two hours before opening, cutting through the middle of the tube, and the pods developed as strongly and produced as many good seeds as those whose flowers were allowed to be visited by bees. The flowers of 0. Lamarckiana, 0. Hookeri, and other large-flowered species, on the contrary, are not fertilized in the buds, but are in need of the help of insects (moths, bees, and bumble-bees). The specific hybrids of this group of Onagra differ in a very conspicuous way from ordinary hybrids in so far as the reciprocal forms often are not identical, but differ widely from one another.2 Although I have observed this fact in numerous cases, it is not a general rule. The hybrid 0. LamarckianaXgigas is identical with 0. gigasX Lamarckiana. In the same way the species of the subgenus Euoenothera crossed with those of Onagra yield uniform hybrids. I crossed the 0. Sellowii with 0. biennis, 0. muricata, and 0. Lamarckiana and observed their reciprocal hybrids to be identical. Within the group of the Onagras, however, the reciprocal hybrids are in most cases different, and with a few exceptions are more similar to their father than to their mother. They are, as it is called, patroclinous. So it is, for instance, with the reciprocal hybrids of 0. muri-2 Die Mutationstheorie 2:47I.