So, if the conditions of life in any community consistently favour a given type of hybrid, whether the favouring conditions are of a cultural or of a material nature, something of a selective trend will take effect in such a community and set toward a hybrid type which shall meet these conditions. The result will be the establishment of a composite pure line showing the advantageous traits of physique and temperament, combined with a varying complement of other characters that have no such selective value. Traits that have no selective value in the given case will occur with fortuitous freedom, combining in unconstrained diversity with the selectively decisive traits, and so will mark the hybrid derivation of this provisionally established composite pure line. With continued intercrossing within itself any given population of such hybrid origin as the European peoples, would tend cumulatively to breed true to such a selectively favourable hybrid type, rather than to any one of the ultimate racial types represented by the parent stocks out of which the hybrid population is ultimately made up. So would emerge a national or local type, which would show the selectively decisive traits with a great degree of consistency but would vary indefinitely in respect of the selectively idle traits comprised in the composite heredity of the population. Such a composite pure line would be provisionally stable only; it should break down when crossed back on either of the parent stocks. This “provisionally stable composite pure line” should disappear when crossed on pure-bred individuals of one or other of the parent stocks from which it is drawn,--pure-bred in respect of the allelomorphic characters which give the hybrid type its typical traits.