The present work is the result of more than twenty years' labour in a limited medical department of biology. It demonstrates once more the truth of the scientific principle, that the truth or falsity of any theory or working hypothesis becomes more and more demonstrable the further its application is attempted in the explanation of new lines of facts. The truth of the degeneracy doctrine had forced itself on the writer long before its popular apotheosis under Lombroso and Nordau, because it alone sufficed for an explanation of constitutional and local defects (encountered in a seemingly limited speciality of medicine), which local causes failed entirely to explain. The investigations thereon resultant have appeared in medical and dental journals for the past two decades. The present work is chiefly based on these researches. At the same time, the author has drawn largely from all fields of biology cultivated by European investigators, while he must acknowledge a particular indebtedness to the investigations (of which he has made large use beside that elsewhere specifically acknowledged) of certain American investigators--Rush, Parkmen, Ray, G. Frank Lydston, C. L. Dana, C. F. Folsom, W. W. Godding, E. C. Spitzka, E. D. Cope, D. R. Brower, Marsh, B. Sachs, Harriet C. B. Alexander, Clara Barrus, H. M. Bannister, Delia E. Howe, Grace Peckham, Adolph Meyer, Kerlin, Wiley, J. G. Kiernan, W. E. Allison, Osborn, R. Dewey, Frederick Peterson, Gihon, Cowles, W. A. Hammond, A. B. Holder, C. H. Hughes, F. W. Starr, F. C. Hoyt, J. H. McBride, C. K. Mills, C. B. Burr, T. D. Crothers, W. S. Christopher, W. X. Sudduth, A. Lagorio, J. Workman, Wilmarth, and others. These scientists had raised an exceedingly stable foundation for the doctrine of degeneracy long before Lombroso and Nordau (forcing one phase of the subject into popular recognition) compelled an examination of the entire doctrine. The work has been written with a special intention of reaching educators and parents. With this object, it has avoided laying stress on any one cause of degeneracy, and ignoring factors which produce it and are aggravated by it. The doctrinaire reformer will here find no support for any limited theory. While it does not pretend in the slightest degree to give all the details of degeneracy, it attempts to lay down general principles for practical purposes in a way that permits their application to the solution of sociologic problems. From a sense of scientific accuracy no attempts have been made to demarcate, rigidly, abnormality from disease, or atavism from arrested development, except as may be done by the features of the cases in which the terms are used. The guiding principle adopted has been that the factors of degeneracy affect in the ancestor the checks on excessive action acquired during the evolution of the race, thus producing a state of nervous exhaustion. The descendant in consequence is unable to reach the state of the ancestor thus nervously exhausted.