animal life on the surface of the earth, yet they are perfectly equivalent, and the balance of the sexes, like the constitution of the atmosphere, depends upon the principles of an unerring intelligence.  You saw in the decline of the Roman empire a people enfeebled by luxury, worn out by excess, overrun by rude warriors; you saw the giants of the North and East mixing with the pigmies of the South and West.  An empire was destroyed, but the seeds of moral and physical improvement in the new race were sown; the new population resulting from the alliances of the men of the North with the women, of the South was more vigorous, more full of physical power, and more capable of intellectual exertion than their apparently ill-suited progenitors; and the moral effects or final causes of the migration of races, the plans of conquest and ambition which have led to revolutions and changes of kingdoms designed by man for such different objects have been the same in their ultimate results—­that of improving by mixture the different families of men.  An Alaric or an Attila, who marches with legions of barbarians for some gross view of plunder or ambition, is an instrument of divine power to effect a purpose of which he is wholly unconscious—­he is carrying a strong race to improve a weak one, and giving energy to a debilitated population; and the deserts he makes in his passage will become in another age cultivated fields, and the solitude he produces will be succeeded by a powerful and healthy population.  The results of these events in the moral and political world may be compared to those produced in the vegetable kingdom by the storms and heavy gales so usual at the vernal equinox, the time of the formation of the seed; the pollen or farina of one flower is thrown upon the pistil of another, and the crossing of varieties of plants so essential to the perfection of the vegetable world produced.  In man moral causes and physical ones modify each other; the transmission of hereditary qualities to offspring is distinct in the animal world, and in the case of disposition to disease it is sufficiently obvious in the human being.  But it is likewise a general principle that powers or habits acquired by cultivation are transmitted to the next generation and exalted or perpetuated; the history of particular races of men affords distinct proofs of this.  The Caucasian stock has always preserved its superiority, whilst the negro or flat-nosed race has always been marked for want of intellectual power and capacity for the arts of life.  This last race, in fact, has never been cultivated, and a hundred generations, successively improved, would be required to bring it to the state in which the Caucasian race was at the time of the formation of the Greek republics.  The principle of the improvement of the character of races by the transmission of hereditary qualities has not escaped the observations of the legislators of the ancient people.  By the divine law of Moses