For a study of the principles of heredity, the reader is referred to Vol. 5 of this series. There it will be learned that heredity is the transmission of germinal characteristics, including germinal weakness. But we do not inherit "disease."

We inherit organization. One may inherit a weak, narrow chest, weak lungs, etc, but he does not inherit tuberculosis. Congenital defects, and larval weaknesses are more likely to be due to nutritional deficiencies than to an affliction of the parent.

There are influences that reach the germ plasm and damage it. Alcohol is the best known of these. Food has a still greater influence on the germ-plasm. Experiments have shown that plants grown in poor soil are smaller and generally produce smaller seeds than those grown in good soil. Further experiments showed that such seed, even when planted in good soil, give rise to smaller plants and seeds than do normal seed. Cold and alcohol have analogous effects upon plants and animals. Fortunately, these changes, being quantitative and never qualitative, do not change the hereditary constitution, and, if their causes are removed, disappear in one to three generations. Indeed Nageli found that plants which had acquired certain adaptive modifications by living on the alpine heights since the "ice age," 'lose these characters perfectly during their first summer in the low lands."

But as Prof. Conklin says, Heredity and Environment, page 247, "probably such cases are not instances of true inheritance; they do not signify a change in the hereditary constitution but an influence on the germ-cells of a nutritive and chemical sort." Such a cause must act through more than one generation to sufficiently deteriorate a hardy strain of germ-plasm to result in a predisposition to "disease."

"No doubt," says Jennings, "it required ages of the continuous action of appropriate causes on definite tissues or sets of organs, to prepare them for the first exhibition of the pneumonia--and these mild at first--of small-pox, measles, and other forms of disease that are denominated specifically contagious. And they who have the misfortune to possess a cachectic or scrofulous habit of body, in consequence of which they are ever liable to serious maladies from slight causes, have not fallen into this dire condition accidentally, or suddenly. If their pedigree could be traced back to a strong healthy race or stock, and all the impairing causes that have been instrumental in reducing them to their unenviable inheritance could be correctly computed and presented, it would exhibit an appalling spectacle."--Philosophy of Human Life, p. 100.