The genetical interpretation of the continuous variation (or indeed any variation) shown by a population, family or group of families requires the use of specifications of two distinct kinds. Firstly, it is necessary to specify the genetical structure of the specifications in suitable terms of the relative frequencies of the various alleles at a locus between the various possible homozygotes and heterozygotes and the distribution of the alleles of different genes in respect of one another. These specifications will depend on the ancestry of the material, the mating system which has been in force, the selection which has been practised (if any), and linkage or other relation of the genes in transmission purposes so long as the relative frequencies of the different possible genotypes can be give, and indeed it is sufficient for many purposes to specify only the average, take over all genes, of the allele frequencies, homozygosis, linkage relations and so on.

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