Fourteen decennial censuses taken since that time (1800 1930) reveal the extraordinary spread of population over the expanding area of the United States' :— Except for the isolated military or trading posts, which always preceded the settlement frontier, the population advanced across the prairie region of the Mid-West with a practically solid front until halted for a time at the eastern edge of the Great Plains. The North and the South sections of the country started out with population growths in the decade 1790-1800 very nearly equal (36.5 and 33.7%) ; but in every succeeding decade before the Civil War the rate of growth of the North was greater, and that of the South less, than its increment in the initial decade. In 1790 the two sections were almost of equal population; by 1890, the population of the North was practically double that of the South. In the decade 1890-1900 the increase of the South exceeded slightly that of the North, owing to the rapid develop ment of the Southern States west of the Mississippi, but in general the increase of the two sections since has been nearly equal.
The temporary halt in the western advance at the edge of the Great Plains was due to the comparative aridity of this region and the discovery of more desirable agricultural valleys and mineral wealth in the mountainous sections and Pacific coast States of the farther west. Population, therefore, leaped over these more barren regions and formed in isolated islands beyond. The first of these overland migrations, which were to become a feature of American life in that generation, was the migration over the Oregon trail to the Oregon country (beginning 1832), the Mormon emigration to Utah (beginning 1847) and the gold rush to California, by sea and the Overland trail (beginning 1848). By the treaty of 1848, ending the war with Mexico, a number of old Spanish settlements in the south-west were added to the population. Discoveries of mineral wealth between 1858 and 188o in Colorado, Nevada, Idaho, Montana and Wyoming created other isolated areas of settlement in these States which spread gradually into the more attractive agricultural valleys of the mountains. Between 188o and 1890 a large part of the Great Plains was taken up. But a small portion of this region was used for farming purposes until after 1900 when the system of dry-farming (q.v.) became generally known. Settlement was then rapid up to the entrance of the United States into the World War, more land being settled than could be held during the period of agricultural depression which followed. Similarly the popu lation of the mountain States has to a large degree followed the rise or fall of prosperity in the mining industries, but the growth of agriculture in these States is proving a stabilizing factor.
In 1920 the centre of population was located 1.9 m. W. of Whitehall in Owen county, Indiana. In 130 years the centre
moved 567 m. westward almost exactly along the 39th parallel of latitude, the extreme north and south variation amounting to only 21.4 miles. In 1930 the centre of population had moved 22.3 miles westward and 7.6 southward to 2.9 miles northeast of Linton, Greene Co., Ind.
In 1790 there were about 600,000 white families in the United States. Speaking broadly there were few very rich and few very poor. Food was abundant. Both social traditions and the religious beliefs of the people encouraged fecundity. The land was but partially settled. Mechanical labour was scarce, and even upon the farm it was difficult to command hired service. Yet only an increased supply of labour made more land or more capital of any practical value, and consequently large families possessed a direct economic advantage. It is estimated that the total immi gration from 1790 to 1830 amounted to but 378,500 persons, thus contributing relatively little to the population increase of nearly 9,000,000. This population increase, which amounted to about 34.5% per decade, was therefore mainly a genetic increase.
From 1830 to the Civil War the rate of increase in the total population of the country continued at the same rate as in the preceding 4o years, the decennial increase for the entire 7o years averaging 34.6% and in no case falling below 32.7%. But after 1830 an increasing proportion of this increase was due to immi gration, the accessions, especially in the decade 184o-5o from Ireland and Germany, being enormous, and the total immigration in that decade rising to 1,427,337 as compared with 538,381 during the preceding decade. It would appear, therefore, that while the natural increase of the native population was actually slowing up, the heavy and increasing immigration concealed this from view at the time. In 185o, 9.7% of the population was foreign born, and in 186o, 13.2%.