Directly observable characters, such as the shape, size, and colour of the body and its parts, were formerly the only means of classifying individuals and populations. They have several disadvantages for this purpose, particularly their complex inheritance, and the fact that almost every character is influenced both by heredity and environment.
Though they have for these reasons largely been superseded for purposes of classification by the blood groups and other hereditary blood characters, it must be borne in mind that they are still the means by which in everyday life we recognize people, and that quantitative observations of them are the only means we have of comparing skeletal material, and observations made on the living before the discovery and application of the blood groups, with people living today. They also present very clear indications of probable natural selection in relation to the environment.
They must therefore continue to be observed with as much precision as possible. It would be a great advantage if their heredity could be more fully understood. It is now clear that almost any one single character, such as stature, is the effect of genes at a considerable number of different loci, so that they are known as polygenic characters.
Attempts have been made, mostly long ago and with little success, to unravel the separate effects of the individual genes involved. The recent very rapid advances made in human genetics as a whole may however be opening the way for a new approach to the genetics of these characters. If their genetics could be understood this would make a major contribution to human biology and classification.()