Tuberculosis is an instance of this; for, while the cause of the disease is the tubercle bacillus, there is enormous difference in the resistance of the body to its action in different individuals.

The disease is to a considerable extent one of families, but while this is true the degree of the influence exerted by heredity can be greatly overestimated.

The disease is so common that in tracing the ancestry of tuberculous patients it is rare to find the disease not represented in the ancestors.

A further difficulty is that the environment is also inherited.

The child of a tuberculous parent has much better opportunity to acquire the infection than a child without such an environment [page 167].