560 THE AMERICAN JOURNAL OF SOCIOLOGY
himself with incomparable facility to all climates, to all condi- tions of life, without losing the fundamental characteristics of his specific unity.
However, this aptitude for variations has some limits. Forty- seven degrees above freezing in the shade at the Senegal, and fifty-six below toward the poles, seem to be the extremes of tem- perature that man can endure ; but this is an enormous variation of 103 degreees. The present human organism is a very ancient heritage and is no longer susceptible of fundamental variations. The human species does not adapt itself to these variations of climate except through a series of graduated and specialized adaptations represented by acclimatization. It is useless to seek by means of hypotheses for the laws of a process that we still see regularly produced under our eyes. Nature always acts in the same way. Man acts only by perfecting the instruments by means of which he has operated and continues to operate.
In this way acclimatization is favored by the internal condi- tions of each society, notably by the power of its capital, by its technical equipment, its scientific power and its moral energy.
In his Trait^ de la variation des animaux et des plantes? Darwin shows how culture and domestication tend to increase the fecundity of plants and animals. This fact had already been observed by Buffon. It is true also of the human species. Sedentary and well-nourished populations have a tendency to develop and to enlarge their limits. This law, therefore, applies as well to the human as to the zoological and botanic zones. Sedentary populations expand without ceasing to be sedentary.
The crossings of different populations favor also expansion by facilitating acclimatization We know that consanguineous unions too long continued are fatal. Crossings constitute favorable con- ditions for the increase of a species, while consanguineous unions, disadvantageous to variability and therefore to adaptation, favor morbid tendencies and end finally in sterility. Here again, how- ever, the most advantageous crossings are those which are gradu- ated. Exogamy, as all sexual unions in general, tends equally to diminish the extreme variations and to bring them back to the
'Vol. II, chap, xvi, pp. 107 ft.
