"Nor is this all. It is not more true, that emigrants, generally speaking, consist of individuals in the prime of life, than that 'they are the most active and vigorous' of that age, as Dr Seybert describes them to be. They are, as it respects the principle at issue, a select class, even compared with that of their own age, generally considered. Their very object in leaving their native countries is to settle in life, a phrase that needs no explanation; and they do so. No equal number of human beings, therefore, have ever given so large or rapid an increase to a community as 'settlers' have invariably done." It is perfectly clear that children are more numerous in the back settlements of America than in the maritime states, not because unoccupied land makes people prolific, but because the most prolific people go to the unoccupied land. Mr Sadler having, as he conceives, fully established his theory of population by statistical evidence, proceeds to prove, "that it is in unison, or rather required by the principles of physiology." The difference between himself and his opponents he states as follows:-- "In pursuing this part of my subject, I must begin by reminding the reader of the difference between those who hold the superfecundity of mankind and myself, in regard to those principles which will form the basis of the present argument. They contend, that production precedes population; I, on the contrary, maintain that population precedes, and is indeed the cause of, production. They teach that man breeds up to the capital, or in proportion to the abundance of the food, he possesses: I assert, that he is comparatively sterile when he is wealthy, and that he breeds in proportion to his poverty; not meaning, however, by that poverty, a state of privation approaching to actual starvation, any more than, I suppose, they would contend, that extreme and culpable excess is the grand patron of population. In a word, they hold that a state of ease and affluence is the great promoter of prolificness. I maintain that a considerable degree of labour, and even privation, is a more efficient cause of an increased degree of human fecundity."