area, and the ratio between the periphery and the internal area continuously decreases as the latter increases. Circumstances of politics and commerce further tend to accentuate that effect, so that for an entire country migration can in certain cases be almost negligible as a factor determining the growth of its population [...]” ( _Ibid_ .)<sup>5</sup> .

States are not only frames of reference for the delimitation of any national population; they also form the actual frame of population data collection. Statistical records thus incorporate migration, implicitly in vital statistics<sup>6</sup> or explicitly in migration statistics. As a result, migration is a matter of interest for demographers, which they take into account as soon as they leave models for tackling real statistics. Their interest in migration can follow two very distinct purposes: either eliminating migration from vital records for its interference with biological demography, or measuring migration for its contribution to overall demography.

Eliminating the interference of external migration with the statistical observation of births and deaths has been an important concern for demographers. Following chemistry<sup>7</sup> , formal demography aims at studying fertility and mortality “in the pure state,” and for this

> 4 Later on, Keyfitz (1968) introduced the notion of   “interacting populations”, and the tool of “immigration vector” in the mathematics of population.

> 5 Interestingly, theories of international migration do not put a greater emphasis on demography than the one demographic theory puts on international migration. Economic theories recognise that international differentials are key factors of migration, but they rarely consider demographic differentials (in population density or population growth) as true factors. Only few of them would endorse a statement such as “modern migration stems mainly from the difference in population growth between the developed and the less developed countries » (Davis 1988 p. 256). In fact, there are too many exceptions to make a rule of it. To take a Middle Eastern example, Lebanon is a country of emigration to Saudi Arabia, despite the fact that Lebanon has a much lower rate of natural population growth than Saudi Arabia (below 1% compared with 3.3% in 1995-2000), and Lebanon is a place of immigration for Syrians, despite the fact that Lebanon has a much higher demographic density than Syria.

> 6 For example, international migration affects death records and consequently the statistical observation of mortality: the death of an emigrant escapes national statistics of countries of origin so that emigration produces the same result as death in reducing the size of a generation.

> 7 Henry (1972) “Analyser, c’est décomposer un tout en ses parties : [...] L’observation nous fournit des données à l’état brut [...] Ces données brutes, qui peuvent paraître simples à un esprit superficiel, sont en réalité le fruit de combinaisons ou de mélanges fort compliqués où interviennent quantités d’éléments [...] Comme en chimie, c’est à l’analyse qu’il revient d’isoler le phénomène à l’état pur [...] Dans nos observations se mêlent l’effet du phénomène qui nous intéresse, la nuptialité, et des phénomènes perturbateurs, mortalité et migrations. » (p. 2021).

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