The causes, that advance or obstruct any one of these three objects, are external or internal. The latter may be divided into physical, civil, and personal, under which last head I comprehend the moral and mechanical habits of mankind. The physical causes are principally climate, soil, and number of subjects; the civil, are government and laws; and political welfare is always in a ratio composed of the force of these particular causes; a multitude of external causes, and all these internal ones, not only control and qualify, but are constantly acting on, and thereby insensibly, as well as sensibly, altering one another, both for the better and the worse, and this not excepting the climate itself.
The powerful efficacy of manners in encreasing a people is manifest from the instance you mention, the Quakers; among them industry and frugality multiply and extend the use of the necessaries of life; to manners of a like kind are owing the populousness of Holland, Switzerland, China, Japan, and most parts of Indostan, &c. in every one of which, the force of extent of territory and fertility of soil is multiplied, or their want compensated by industry and frugality.
Neither nature nor art have contributed much to the production of subsistence in Switzerland, yet we see frugality preserves and even increases families, that live on their fortunes, and which, in England, we call the gentry; and the observation we cannot but make in the southern part of this kingdom, that those families, including all superior ones, are gradually becoming extinct, affords the clearest proof, that luxury (that is, a greater expence of subsistence than in prudence a man ought to consume) is as destructive as a proportionable want of it; but in Scotland, as in Switzerland, the gentry, though one with another they have not one-fourth of the income, increase in number.
And here I cannot help remarking, by the by, how well founded your distinction is between the increase of mankind in old and new settled countries in general, and more particularly in the case of families of condition. In America, where the expences are more confined, to necessaries, and those necessaries are cheap, it is common to see above one hundred persons descended from one living old man. In England, it frequently happens, where a man has seven, eight, or more children, there has not been a descendant in the next generation, occasioned by the difficulties the number of children has brought on the family, in a luxurious dear country, and which have prevented their marrying.
That this is more owing to luxury than mere want appears from what I have said of Scotland, and more plainly from parts of England remote from London, in most of which the necessaries of life are nearly as dear, in some dearer than London, yet the people of all ranks marry and breed up children.
Again; among the lower ranks of life, none produce so few children as servants. This is, in some measure, to be attributed to their situation, which hinders marriage, but is also to be attributed to their luxury and corruption of manners, which are greater than among any other set of people in England, and is the consequence of a nearer view of the lives and persons of a superior rank, than any inferior rank, without a proper education, ought to have.
The quantity of subsistence in England has unquestionably become greater for many ages; and yet if the inhabitants are more numerous, they certainly are not so in proportion to our improvement of the means of support. I am apt to think there are few parts of this kingdom, that have not been at some former time more populous than at present. I have several cogent reasons for thinking so of great part of the counties I am most intimately acquainted with; but as they were probably not all most populous at the same time, and as some of our towns are visibly and vastly grown in bulk, I dare not suppose, as judicious men have done, that England is less peopled than heretofore.
This growth of our towns is the effect of a change of manners, and improvement of arts, common to all Europe; and though it is not imagined, that it has lessened the country growth of necessaries, it has evidently, by introducing a greater consumption of them, (an infallible consequence of a nation's dwelling in towns) counteracted the effects of our prodigious advances in the arts.
But however frugality may supply the place, or prodigality counteract the effects, of the natural or acquired subsistence of a country, industry is, beyond doubt, a more efficacious cause of plenty than any natural advantage of extent or fertility. I have mentioned instances of frugality and industry united with extent and fertility. In Spain and Asia Minor, we see frugality joined to extent and fertility, without industry; in Ireland, we once saw the same; Scotland had then none of them but frugality. The change in these two countries is obvious to every one, and it is owing to industry not yet very widely diffused in either. The effects of industry and frugality in England are surprising; both the rent and the value of the inheritance of land depend on them greatly more than on nature, and this, though there is no considerable difference in the prices of our markets. Land of equal goodness lets for double the rent of other land lying in the same county, and there are many years purchase difference between different counties, where rents are equally well paid and secure.
Thus manners operate upon the number of inhabitants, but of their silent effects upon a civil constitution, history, and even our own experience, yields us abundance of proofs, though they are not uncommonly attributed to external causes: their support of a government against external force is so great, that it is a common maxim among the advocates of liberty, that no free government was ever dissolved, or overcome, before the manners of its subjects were corrupted.