A few abnormalities, such as a premature graying of the hair (one family cited by K. Pearson) are well enough attested to be admitted. Many others, such as baldness, are probably Mendelian but not yet sufficiently supported by evidence.
None of these characters, it will be observed, is of much significance eugenically. If the exact manner of inheritance of some of the more important mental and physical traits were known, it would be of value. But it is not a prerequisite for eugenic action. Enough is known for a working program.
To sum up: the features in the modern view of heredity, which the reader must keep in mind, are the following:
- That the various characters which make up the physical constitution of any individual plant or animal are due to the action (concurrently with the environment, of course) of what are called, for convenience, factors, separable hypothetical units in the germ-plasm, capable of independent transmission.
- That each visible character is due to the coöperative action of an indefinitely large number of factors; conversely, that each of these factors affects an indefinitely large number of characters.