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.
Bateson, in 1894, showed that symmetry is a characteristic common to all organisms. This may affect the whole or parts of an organ. Major symmetry involves the whole organism and minor symmetry only an organ or part. There are meristic variations, involving the symmetrical pattern, and substantive variations involving changes in the constitution or substance of the organism. Red-flowering plants, for example, may yield offspring bearing white flowers. Substantive variations are often discontinuous, or accidental, and are infrequent.
Organic bodies are built up of a number of cells. The living material of cells is protoplasm formed out of many elements, of which carbon, oxygen, hydrogen, nitrogen, and sulphur are the more important. New cells arise from bipartition of existing cells. Therefore by following back the history of any animal or plant we will arrive at a stage when its ancestors had only one cell. Every animal or plant which is propagated sexually actually starts as a cell and develops through its main evolutionary changes in the embryonic state. Cells are liable to all the evolutionary changes that the organism as a whole is subject to.
Studies of embryology have shown that the fusion of biparental reproductive cells results in the formation of a complete new individual which, at the time of the fusing of the two conjugating cells, called gametes, or germ cells, inherits the characteristics of each parent and its ancestors.