This process, which is plain to see on the largest scale throughout living nature, may be more minutely studied, as it acts within a narrower range, in the human species. Here we statistically formulate it in the terms of birth-rate and death-rate; by the mutual relationship of the two courses of the birth-rate and the death-rate we are able to estimate the evolutionary rank of a nation, and the degree in which it has succeeded in subordinating the primitive standard of quantity to the higher and later standard of quality. It is especially in Europe that we can investigate this relationship by the help of statistics which in some cases extend for nearly a century back. We can trace the various phases through which each nation passes, the effects of prosperity, the influence of education and sanitary improvement, the general complex development of civilisation, in each case moving forward, though not regularly and steadily, to higher stages by means of a falling birth-rate, which is to some extent compensated for by a falling death-rate, the two rates nearly always running parallel, so that a temporary rise in the birth-rate is usually accompanied by a rise in the death-rate,—by a return, that is to say, to the conditions which we find at the beginning of animal life,—and a steady fall in the birth-rate is always accompanied by a fall in the death-rate. The modern phase of this movement, soon after which our precise knowledge begins, may be said to date from the industrial expansion, due to the introduction of machinery, which Professor Marshall places in England about the year 1760. That represents the beginning of an era in which all civilised and semi-civilised countries are still living. For the earlier centuries we lack precise data, but we are able to form certain probable conclusions. The population of a country in those ages seems to have grown very slowly and sometimes even to have retrograded. At the end of the sixteenth century the population of England and Wales is estimated at five millions and at the end of seventeenth at six millions,—only 20% increase during the century—although during the nineteenth century the population nearly quadrupled. This very gradual increase of the population seems to have been by no means due to a very low birth-rate, but to a very high death-rate. Throughout the Middle Ages a succession of virulent plagues and pestilences devastated Europe. Smallpox, which may be considered the latest of these, used to sweep off large masses of the youthful population in the eighteenth century. The result was a certain stability and a certain well-being in the population as a whole, these conditions being, however, maintained in a manner that was terribly wasteful and distressing.