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.
In such a case we call the character A a compound allelomorph, and we can speak of the integral characters which constitute it as hypallelomorphs. We ought to write the heterozygote (A A′ A″ . . .) B and the gametes produced by it may be of the form A, A′, A″, A‴, . . . B. Or the resolution may be incomplete in various degrees, as we already suspect from certain instances; in which case we may have gametes A, A′ A″, A‴ A″″, A′ A″ Av, . . . B, and so on. Each of these may meet a similar or a dissimilar gamete in fertilisation, forming either a homozygote, or a heterozygote with its distinct properties.
In the case of compound allelomorphs we know as yet nothing of the statistical relations of the several gametes.
Thus we have the conception
(5) of a Compound character, borne by one gamete, transmitted entire as a single character so long as fertilisation only occurs between like gametes, or is, in other words, “symmetrical,” but if fertilisation take place with a dissimilar gamete (or possibly by other causes), resolved into integral constituent characters, each separately transmissible.
Next, as, by the union of the gametes bearing the various hypallelomorphs with other such gametes, or with gametes bearing simple allelomorphs, in fertilisation, a number of new zygotes will be formed, such as may not have been seen before in the breed: these will inevitably be spoken of as varieties; and it is difficult not to extend the idea of variation to them. To distinguish these from other variations—which there must surely be—we may call them
(6) Analytical variations in contradistinction to
(7) Synthetical variations, occurring not by the separation of pre-existing constituent-characters but by the addition of new characters.
Lastly, it is impossible to be presented with the fact that in Mendelian cases the cross-bred produces on an average equal numbers of gametes of each kind, that is to say, a symmetrical result, without suspecting that this fact must correspond with some symmetrical figure of distribution of those gametes in the cell-divisions by which they are produced.