After man—through the evolution of the structures and faculties which distinguish him from the lower animals, the large brain, with its accompanying memory, the organs of speech, the hand, the erect attitude—had achieved the conquest of the earth, his selection and evolution along the ancestral lines gradually diminished, and has now almost ceased. At the present day clever, strong, or active people do not on the average have an appreciably more numerous progeny than those who are not exceptionally endowed. No modern race is intellectually superior to the Greeks who flourished more than two thousand years ago. The brains, the hands, the organs of speech, the erect attitude, have not altered. Apparently nothing more than traditional knowledge has improved.
The gradual accumulation of traditional knowledge during prehistoric times enabled man to cultivate animals and plants, and so to increase and regulate his supply of food. As a consequence his numbers multiplied. Areas of country which formerly supported only a few wandering hunters now afforded sustenance to growing multitudes of agriculturists, who often dwelt together for mutual protection in villages. Commerce followed agriculture, towns and cities arose, and civilisation dawned.
Civilisation implies a dense and settled community, protected from most of the dangers which beset wild animals, and in which, therefore, the elimination of the unfit is no longer of the kind that weeded out the brute and the utter savage. Some sort of elimination does occur, however, for, even in the most civilised states, multitudes of people perish in youth, before they have contributed their full quota of offspring to the race.
Natural Selection at Work
We have excellent opportunities of studying this elimination and noting whether it results in evolution. Indeed, man presents the only instance in Nature in which we are able to observe natural selection actually at work. In all modern states statistics are compiled which set out the causes of death, the mortality from each cause, and the ages of its victims. By comparing races which have been much afflicted by this or that cause of mortality with races that have been little or not at all affected, we are able to ascertain the resulting racial change, if any. As may be noted by everyone, civilised people perish, with rare exceptions, of disease.
MANKIND’S LONG BATTLE AGAINST BACTERIA
Resistance of Races to Disease
W
WE have just seen that every race is resistant to every disease precisely in proportion to its past experience of it. It follows that the evolution of civilised peoples is against disease. If any other kind of evolution is now occurring, no one as yet has been able to demonstrate it, though many unproved guesses have been made. Mere alterations in traditional knowledge is not evolution. Children may derive it just as well from other people as from their parents.
The vast majority of deaths from disease are of zymotic origin. A zymotic or microbic disease is caused by the entrance into the body of minute animals or plants (microbes), which find their nutriment there. There are many species of microbes, each disease being due to one. Some species are mainly air-borne, and infect through the breath; others are water-borne; others earth-borne; yet others insect-borne; while a few pass by actual contact from an infected to a healthy person.