Under the Mendelian theories of heredity some qualification of these broad generalisations is called for. As has already been noted above, the peoples of Europe, each and several, are hybrid mixtures made up of several racial stocks. The like is true in some degree of most of the peoples outside of Europe; particularly of the more important and better known nationalities. These various peoples show more or less distinct and recognisable national types of physique--or perhaps rather of physiognomy--and temperament, and the lines of differentiation between these national types incontinently traverse the lines that divide the racial stocks. At the same time these national types have some degree of permanence; so much so that they are colloquially spoken of as types of race. While no modern anthropologist would confuse nationality with race, it is not to be overlooked that these national hybrid types are frequently so marked and characteristic as to simulate racial characters and perplex the student of race who is intent on identifying the racial stocks out of which any one of these hybrid populations has been compounded. Presumably these national and local types of physiognomy and temperament are to be rated as hybrid types that have been fixed by selective breeding, and for an explanation of this phenomenon recourse is to be taken to the latterday theories of heredity. To any student familiar with the simpler phenomena of hybridism it will be evident that under the Mendelian rules of hybridisation the number of biologically successful--viable--hybrid forms arising from any cross between two or more forms may diverge very widely from one another and from either of the parent types. The variation must be extreme both in the number of hybrid types so constructed and in the range over which the variation extends,--much greater in both respects than the range of fluctuating (non-typical) variations obtainable under any circumstances in a pure-bred race, particularly in the remoter filial generations. It is also well known, by experiment, that by selective breeding from among such hybrid forms it is possible to construct a composite type that will breed true in respect of the characters upon which the selection is directed, and that such a “pure line” may be maintained indefinitely, in spite of its hybrid origin, so long as it is not crossed back on one or other of the parent stocks, or on a hybrid stock that is not pure-bred in respect of the selected characters.