many generations, weeding out the elements of the fair race and determining an ever increasing predominance of _Homo Alpinus_. In order to understand the importance of these internal selections, it is necessary to realise that their effects are cumulative in a high degree, when the same influences continue to work through many generations. Thus, if within any people there are two equally numerous classes of persons of different mental constitution, _A_ and _B_, and if these constitutions determine that the one group _A_ has a net birth rate of three children per pair of adults, while the other _B_ has a birth rate of four per pair of parents; then, in the third generation after one century, the numbers of the two classes, other things being the same for both, will be as ten to sixteen. After two centuries the one class will be more than twice as numerous as the other; and after three centuries the numbers of the class _A_ will constitute about fifteen per cent, only of the whole population. Late marriage is also very important. Suppose that of two classes, _A_ marries at 35, and _B_ at 25 years, and that each produces four children per marriage; then (other things being the same) after three and a half centuries _B_ becomes four times as numerous as _A_. These two factors generally work together. But, apart from the change of racial composition of a heterogeneous nation by internal selection of this sort, changes of the constitution of even a racially homogeneous people may be produced through selection affecting persons of particular mental tendencies. One of the most striking instances of this is the elimination of the religious tendencies from the constitution of a people by negative selection through the action of the Roman Church[129]. For many centuries the Roman Church has attracted to her service very large numbers of those who were by nature most religiously minded, and it has imposed celibacy upon them, it has forbidden them to transmit their natural piety to descendants. In Protestant countries this process of negative selection of the religious tendencies was continued for a much briefer period than in the Catholic countries. It is maintained with much plausibility that we may see the result in the fact that sincere and natural piety is far commoner in the Protestant countries than in the Roman Catholic; that in the two countries Italy and Spain, in which the influence of the Roman Church has been greater than in any others, the people are now the least religiously minded of any in Europe; that with them religion has become purely formal and external, that the mass of the people, though outwardly conforming, is absolutely irreligious; that in fact this form of religion tends to exterminate itself in the long run by insisting upon that form of reproductive selection[130].