Each such character, which is capable of being dissociated or replaced by its contrary, must henceforth be conceived of as a distinct _unit-character_; and as we know that the several unit-characters are of such a nature that any one of them is capable of independently displacing or being displaced by one or more alternative characters taken singly, we may recognize this fact by naming such unit-characters _allelomorphs_. So far, we know very little of any allelomorphs existing otherwise than as _pairs_ of contraries, but this is probably merely due to experimental limitations and the rudimentary state of our knowledge. In one case (combs of fowls) we know three characters, _pea_ comb, _rose_ comb and _single_ comb; of which _pea_ and _single_, or _rose_ and _single_, behave towards each other as a pair of allelomorphs, but of the behaviour of _pea_ and _rose_ towards each other we know as yet nothing. We have no reason as yet for affirming that any phenomenon properly described as _displacement_ of one allelomorph by another occurs, though the metaphor may be a useful one. In all cases where _dominance_ has been perceived, we can affirm that the members of the allelomorphic pair stand to each other in a relation the nature of which we are as yet wholly unable to apprehend or illustrate. To the new conceptions already enumerated we may therefore add (4) _Unit-characters_ of which some, _when once arisen by Variation_, are alternative to each other in the constitution of the gametes, according to a definite system. From the relations subsisting between these characters, it follows that as each zygotic union of allelomorphs is _resolved_ on the formation of the gametes, no zygote can give rise to gametes collectively representing more than _two_ characters allelomorphic to each other, apart from new variation. From the fact of the existence of the interchangeable characters we must, for purposes of treatment, and to complete the possibilities, necessarily form the conception of an _irresoluble base_, though whether such a conception has any objective reality we have no means as yet of determining. We have now seen that when the varieties _A_ and _B_ are crossed together, the heterozygote, _AB_, produces gametes bearing the pure _A_ character and the pure _B_ character. In such a case we speak of such characters as _simple_ allelomorphs. In many cases however a more complex phenomenon happens. The character brought in on fertilisation by one or other parent may be of such a nature that when the zygote, _AB_, forms its gametes, these are not individually bearers merely of _A_ and _B_, _but of a number of characters themselves again integral_, which in, say _A_, behaved as one character so long as its gametes united in fertilisation with others like themselves, but on cross-fertilisation are resolved and redistributed among the gametes produced by the cross-bred zygote.