The tendency of evolution to-day is to effect a decline in the birth-rate. In the future far more attention will be paid, than has been paid in the past, to the demand of social hygiene that potential parents shall be careful to procreate healthy children only. On the other hand, the knowledge that will enable parents to prevent undesired conceptions will become more and more widely diffused. In times to come, an ever-increasing proportion of pregnancies will be deliberately willed. The decline in the birth-rate will necessarily result in a decline in the death-rate, and more especially in a decline in the death-rate of infants and children. Ultimately, we shall see a decline, not merely in the birth-rate and the death-rate, but also in the difference between the total number of the births and of the deaths. It is beyond dispute that these figures are tending to become less variable and more constant than they were in former times. CHAPTER II STATISTICAL PROBLEMS OF POPULATION _Miscarriages, Premature Births, and Still-Births._--The statistical data regarding miscarriages, premature births, and still-births are somewhat untrustworthy. There is no general agreement as to the precise signification of these terms among the medical practitioners, coroners, and registrars of any one country--not to speak of differences in these matters in different countries; and this difficulty applies above all to the matter of still-births. In some countries, children dying during the act of birth or within a few hours after birth are regarded as born alive, but in others as still-born. When we are comparing the birth-rate and the death-rate in different countries, these causes of error must not be forgotten. In the twentieth century, in the civilised countries of Europe, the premature births vary between 5 and 9 per cent., and the still-births between 3 and 4 per cent., of all births. For every 100 still-born girls there are approximately 130 still-born boys. Among the lower classes of the population, still-births are more frequent than among the upper classes. Within the same class, such births are more frequent among those living in unfavourable conditions than among those more favourably situated; and in manufacturing towns they are more frequent than in agricultural districts. The proportion is affected by the age of the mother, and still-births are at a minimum among mothers at ages from 20 to 25 years. In the course of time, notwithstanding the gigantic development of manufacturing industry, and in spite of the more accurate registration of still-births, the proportion of such births has diminished; the principal reason for this is the advance in medical science.