For 1974 and earlier, the birth rate is calculated as the total number of live births divided by the population aged 20 to 39. The marriage rate is calculated as the total number of marriages divided by the population aged 20 to 39. Sources: Ministry of Internal Affairs and Communications, "Labour Force Survey"; Ministry of Health, Labour and Welfare, "Vital Statistics."
You need to be prescient. I do not particularly espouse the Darwinian theory. But Mr. Darwin may have been on to something when he said, “that which will survive is the species which is most adaptable to change.” (Please pardon literary freedom for my paraphrase).