Such facts thus set out for the first cross-bred generation may without doubt be predicated for subsequent generations.
What then is the significance of the facts?
Analysis of exceptions.
Assuming that all these “contradictory” phenomena happened truly as alleged, and were not pathological or due to error—an explanation which seems quite inadequate—there are at least four possible accounts of such diverse results—each valid, without any appeal to ancestry.
1. That dominance may exceptionally fail—or in other words be created on the side which is elsewhere recessive. For this exceptional failure we have to seek exceptional causes. The artificial creation of dominance (in a character usually recessive) has not yet to my knowledge been demonstrated experimentally, but experiments are begun by which such evidence may conceivably be obtained.
2. There may be what is known to practical students of evolution as the false hybridism of Millardet, or in other words, fertilisation with—from unknown causes—transmission of none or of only some of the characters of one pure parent. The applicability of this hypothesis to the colours and shapes of peas is perhaps remote, but we may notice that it is one possible account of those rare cases where two pure forms give a mixed result in the first generation, even assuming the gametes of each pure parent to be truly monomorphic as regards the character they bear. The applicability of this suggestion can of course be tested by study of the subsequent generations, self-fertilised or fertilised by similar forms produced in the same way. In the case of a genuine false-hybrid the lost characters will not reappear in the posterity.
3. The result may not be a case of transmission at all as it is at present conceived, but of the creation on crossing of something new. Our AB’s may have one or more characters peculiar to themselves. We may in fact have made a distinct “mule” or heterozygote form. Where this is the case, there are several subordinate possibilities we need not at present pursue.
4. There may be definite variation (distinct from that proper to the “mule”) consequent on causes we cannot yet surmise (see pp. [125] and 128).
The above possibilities are I believe at the present time the only ones that need to be considered in connexion with these exceptional cases[75]. They are all of them capable of experimental test and in certain instances we are beginning to expect the conclusion.
The “mule” or heterozygote.