The studies of variations in plants by mutations, made by the Dutch botanist De Vries, have opened up wide fields of study regarding the causes of variation. He has shown that increased bulk or better coloration may result from improved nutrition and more light, and that such improved characteristics may be inherited.
A law of ancestral heredity has been worked out for men by biometricians, and this has been confirmed by the experiments of Professor Johannsen, of Copenhagen, on self-fertilizing beans, and by Jennings on protozoa. This hypothesis suggests that every ancestor of a particular man or woman contributes its quota to the heritable qualities displayed by that individual. The average amount of resemblance between an individual and any of his particular ancestors is capable of being numerically expressed.
The experiments and conclusions of Gregor Mendel (1822-1882) tend to oppose the law of ancestral heredity, but it is believed that any exceptional cases may be explained by the operation of special conditions.
Karl Pearson has shown by the analyses of numerous statistical records of Englishmen that by artificial selection any selected characteristic, such as facial contour or stature, can be changed within a few generations. But when the character has been changed about 90 per cent within a short time another method must be employed, because the original one then becomes less efficient.
Individuals in any given population who differ in size from the mean of the population give rise to offspring which differs from that mean value in the same direction but to a smaller extent. The same law applies to the color of the hair or to intelligence or constitution. Selection will always produce a change in the average character of a population taken as a whole. But selection within a pure line, or one which shows only normal variability about a mean or type value, does not produce marked changes.
The usual selection within any particular population consists in the partial separation of extreme types.
The personal characteristics of any ancestor do not influence his descendants. Only the typical characteristics are handed down.
These and many other facts developed by investigations in biometry should be of value in regulating immigration, so as to guard against degenerative influences, and they have greatly increased the efficiency of farming by showing how to improve farm stocks and crops so as to yield larger returns. Farmers have been more ready than politicians to avail of their advantages. We note how the speed of racing and trotting horses, and the milking capacity of cows, have been improved by the past century, but we are doing little to reform national health and efficiency.
Mutation is the name given to the process of origination of a new species or character accomplished by a single step or by a series of steps.