Now it can be explained, I believe for the first time, how it happens that man, unlike the lower animals, is not limited to a particular period of the year in which to court the female.[181] The Darwinian theory of natural selection can, as it seems to me, account for the periodicity of the sexual instinct in such a rude race as the Western Australians, among whom the mortality of children is so enormous that the greater number of them do not survive even the first month after birth,[182] and who inhabit a land pre-eminently unproductive of animals and vegetables fitted to sustain human life, a land where, “during the summer seasons, the black man riots in comparative abundance, but during the rest of the year ... the struggle for existence becomes very severe.”[183] The more progress man makes in arts and inventions; the more he acquires the power of resisting injurious external influences; the more he rids himself of the necessity of freezing when it is cold, and starving when nature is less lavish with food; in short, the more independent he becomes of the changes of the seasons—the greater is the probability that children born at one time of the year will survive as well, or almost as well, as those born at any other. Variations as regards the pairing time, always likely to occur occasionally, will do so the more frequently on account of changed conditions of life, which directly or indirectly cause variability of every kind;[184] and these variations will be preserved and transmitted to following generations. Thus we can understand how a race has arisen, endowed with the ability to procreate children in any season. We can also understand how, even in such a rude race as the Yahgans in Tierra del Fuego, the seasonable distribution of births seems to be pretty equal, as there is, according to the Rev. T. Bridges, “such a variety of food in the various seasons that there is strictly no period of hardship, save such as is caused by accidents of weather.” We can explain, too, why the periodical fluctuation in the number of births, though comparatively inconsiderable in every civilized society, is greater in countries predominantly agricultural, such as Chili, than in countries predominantly industrial, as Saxony;[185] why it is greater in rural districts than in towns;[186] and why it was greater in Sweden in the middle of the last century than it is now.[187] For the more man has abandoned natural life out of doors, the more luxury has increased and his habits have got refined, the greater is the variability to which his sexual life has become subject, and the smaller has been the influence exerted upon it by the changes of the seasons.