The population of Great Britain increases so as to double itself in about fifty years.

To do this it is evident that each married couple must average three children who live to be married at the age of about twenty-five.

Add to these those who die in infancy, those who never marry, or those who marry late in life and have no offspring, the number of children born to each marriage must average four or five, and we know that families of seven or eight are very common, and of ten and twelve by no means rare.

But from inquiries at almost every Dyak tribe I visited, I ascertained that the women rarely had more than three or four children, and an old chief assured me that he had never known a woman to have more than seven.
In a village consisting of a hundred and fifty families, only one consisted of six children living, and only six of five children, the majority of families appearing to be two, three, or four.

Comparing this with the known proportions in European countries, it is evident that the number of children to each marriage can hardly average more than three or four; and as even in civilized countries half the population die before the age of twenty-five, we should have only two left to replace their parents; and so long as this state of things continued, the population must remain stationary.