MB × BM
gives exclusively offspring of muricata type. Evidently, apart from all controversy as to the significance of the "mutants" of Lamarckiana, we have here a series of observations of the first importance.
The fact that reciprocal crossings give constantly distinct results must be taken to indicate that the male and female sides of one, if not of both, of the parents are different in respect of characters which they bear. This is de Vries's view, and he concludes rightly, I think, that the evidence from all the experiments shows that both biennis and muricata are in this condition, having one set of characters represented in their pollen-grains and another in their ovules. The plants breed true, but their somatic structures are compounded of the two sets of elements which pass into them from their maternal and paternal sides respectively. This possibility that species may exist of which the males really belong to one form and the females to another, is one which it was evident from the first announcement of the discovery of Mendelian segregation might be found realised in nature.[5]
Oe. biennis and muricata were crossed reciprocally with each other and with a number of other species, and the behaviour of each, when used as mother, was consistently different from its behaviour when used as father. De Vries is evidently justified by the results of this series of experiments in stating that the "Bild," as he terms it, or composition of the male and female sides of these two species, biennis and muricata, are distinct. On the evidence before us it is not, however, possible to form a perfectly clear idea of each, and until details are published, a reader without personal knowledge of the material cannot do more than follow the general course of the argument. For fuller comprehension a proper analysis of the characters with a clear statement of how they are distributed among the several types and crosses is absolutely necessary. According to de Vries the female of biennis possesses a group of characters which he defines as "conica" in allusion to the shape of the flower-buds. Besides the conical buds, this group of features includes imperfect development of wood, rendering the plant very liable to attacks of Botrytis, and comparatively narrow leaves.
The female of muricata carries a group of features which he calls "frigida," and, though this is not quite explicitly stated in a definition of that type, it is to be inferred[6] that its characteristics are regarded as greater height, strong development of wood with comparative resistance to Botrytis, and broad leaves.
The characters borne by the male parts of the two species are in general those by which they are outwardly distinguished. For example, the leaves of Oe. biennis are comparatively broad and are bright green, while those of muricata are much narrower and of a glaucous green, and I understand that de Vries regards these properties as contributed by the male side in each case and to be carried by the male cells of each species. The suggestion as regards biennis and muricata comes near the conception often expressed by naturalists in former times (e. g., Linnaeus) and not rarely entertained by breeders at the present day, that the internal structure is contributed by the mother and the external by the father.
On the other hand, the offspring of each species when used as mother is regarded as possessing in the main the features of the maternal "Bild," but the matter is naturally complicated by the introduction of features from the father's side, and it is here especially that the account provided is at present unsatisfactory and inconclusive. There seems, however, to be no serious doubt that biennis and muricata each in their outward appearance exhibit on the whole the features which their pollens respectively carry, and that the features borne by their ovules are in many respects distinct.
The types are thus "hybrids" which breed true. The results of intercrossing them each way are again "hybrids" which breed true. It will be remembered that on former occasions de Vries has formulated a general rule that species-hybrids breed true, but that the cross-breds raised by interbreeding varieties do not. One of these very cases was quoted[7] as an illustration of thisprinciple, viz: muricata × biennis. The grounds for this general statement have always appeared to me insufficient, and with the further knowledge which the new evidence provides we are encouraged to hope that when a proper factorial analysis of the types is instituted we shall find that the phenomenon of a constant hybrid will be readily brought into line with the systems of descent already worked out for such cases as that of the Stocks, and others already mentioned.
In further discussion of these facts de Vries makes a suggestion which seems to me improbable. Since the egg-cells of muricata, for instance, bear a certain group of features which are missing on the male side, and conversely the pollen bears features absent from the female side, he is inclined to regard the bad pollen grains as the bearers of the missing elements of the male side and to infer that there must similarly be defective ovules representing the missing elements of the female side. No consideration is adduced in support of this view beyond the simple fact that the characters borne by male and female are dissimilar, whereas it would be more in accord with preconception if the same sets of combinations were represented in each—as in a normal Mendelian case. There is as yet no instance in which the absence of any particular class of gametes has been shown with any plausibility to be due to defective viability, though there are, of course, cases in which certain classes of zygotes do not survive owing to defective constitution (e. g., the albinos of Antirrhinum studied by Baur, and the homozygous yellow mice). I am rather inclined to suppose that in these examples of hybrids breeding true we shall find a state of things comparable with that to which we formerly applied the terms "coupling" and "repulsion." In these cases certain of the possible combinations of factors occur in the gametic series with special frequency, being in excess, while the gametes representing other combinations are comparatively few. In a recent paper on these cases Professor Punnett and I have shown that these curious results vary according to the manner in which the factors are grouped in the parents. If A and B are two factors which exhibit these phenomena we find that the gametic series of the double heterozygote differs according as the combination is made by crossing AB × ab, or by crossing AB × aB. In a normal Mendelian case the F1 form, AaBb, produces gametes AB, Ab, aB, ab, in equal numbers; but in these peculiar cases those gametes which contain
| Gametic | series | # of gametes | # of zygotes | ||||
| AB | Ab | aB | ab | in series | in series | ||
| 1 | (n-1) | (n-1) | 1 | 2n | 4n2 | ||
| Partial repulsion | { | 1 | 31 | 31 | 1 | 64 | 4096 |
| from zygote | { | 1 | 15 | 15 | 1 | 32 | 1024 |
| of form | { | 1 | 7 | 7 | 1 | 16 | 256 |
| Ab × aB | { | 1 | 3 | 3 | 1 | 8 | 64 |
| 1 | 1 | 1 | 1 | 4 | 16 | ||
| { | 3 | 1 | 1 | 3 | 8 | 64 | |
| Partial coupling | { | 7 | 1 | 1 | 7 | 16 | 256 |
| from zygote | { | 15 | 1 | 1 | 15 | 32 | 1024 |
| of form | { | 31 | 1 | 1 | 31 | 64 | 4096 |
| AB × ab | { | 63 | 1 | 1 | 63 | 128 | 16384 |
| { | (n-1) | 1 | 1 | (n-1) | 2n | 4n2 | |
| Nature | of | zygotic | series | ||||
| AB | Ab | aB | ab | ||||
| Partial repulsion | { | 2n2+1 | n2-1 | n2-1 | 1 | ||
| from zygote | { | 2049 | 1023 | 1023 | 1 | ||
| of form | { | 513 | 255 | 255 | 1 | ||
| Ab × aB | { | 33 | 15 | 15 | 1 | ||
| 9 | 3 | 3 | 1 | ||||
| { | 41 | 7 | 7 | 9 | |||
| Partial coupling | { | 177 | 15 | 15 | 49 | ||
| from zygote | { | 737 | 31 | 31 | 225 | ||
| of form | { | 3009 | 63 | 63 | 961 | ||
| AB × ab | { | 12161 | 127 | 127 | 3969 | ||
| { | (3n2-(2n-1) | (2n-1) | (2n-1) | n2-(2n-1) |