If an organism producing germ-cells of a given constitution, uniform in respect of the characters they bear, breeds with another organism[13] bearing _precisely similar_ germ-cells, the offspring resulting will, if the conditions are identical, be uniform. [13] For simplicity the case of self-fertilisation is omitted from this consideration. In practice such a phenomenon is seen in _pure_-breeding. It is true that we know no case in nature where all the germ-cells are thus identical, and where no variation takes place beyond what we can attribute to conditions, but we know many cases where such a result is approached, and very many where all the essential features which we regard as constituting the characters of the breed are reproduced with approximate certainty in every member of the pure-bred race, which thus closely approach to uniformity. But if two germ-cells of dissimilar constitution unite in fertilisation, what offspring are we to expect[14]? First let us premise that the answer to this question is known experimentally to differ for many organisms and for many classes of characters, and may almost certainly be in part determined by external circumstances. But omitting the last qualification, certain principles are now clearly detected, though what principle will apply in any given case can only be determined by direct experiment made with that case. [14] In all the cases discussed it is assumed that the gametes are similar except in regard to the “heritage” they bear, and that no _original_ variation is taking place. The case of mosaics is also left wholly out of account (see later). This is the phenomenon of _cross_-breeding. As generally used, this term means the union of members of dissimilar varieties, or species: though when dissimilar gametes[15] produced by two individuals of the same variety unite in fertilisation, we have essentially _cross_-breeding in respect of the character or characters in which those gametes differ. We will suppose, as before, that these two gametes bearing properties unlike in respect of a given character, are borne by different individuals. [15] The term “gamete” is now generally used as the equivalent of “germ-cell,” whether male or female, and the term “zygote” is here used for brevity to denote the organism resulting from fertilisation. In the simplest case, suppose a gamete from an individual presenting any character in intensity _A_ unite in fertilisation with another from an individual presenting the same character in intensity _a_. For brevity’s sake we may call the parent individuals _A_ and _a_, and the resulting zygote _Aa_. What will the structure of _Aa_ be in regard to the character we are considering?