Even when well-marked disease is absent in the parents, exhausting and debilitating influences, age at time of conception and overwork, may play a disastrous part. Dr. Langdon Down (_Mental Diseases of Childhood_) has shown how the same influences play a part in the production of idiocy; how, for instance, a man may during periods of strain and overwork conceive idiot children, and at other periods healthy children. Marro has made some interesting investigations into the ages of the father at the period of conception of criminals, as compared with ordinary persons and with the insane. He divided the fathers into three groups, according to age at conception: the first included those in the period of immaturity, which he reckoned as below 25 years of age; the second was the period of maturity from 26 to 40; the third from 41 onwards, the period of decadence. Plate VII. represents in a graphic form the percentage of fathers belonging to each period in various groups; the first column in each group representing the proportion of fathers belonging to the period of immaturity, the second those belonging to the period of maturity, the third those belonging to the period of decadence. It will be seen that the largest proportion of immature parents is among the class of thieves, although among the insane the proportion is still larger. More remarkable is the abnormally large proportion of criminals with parents belonging to the period of decadence. It is most marked among the murderers, 52.9 per cent. of whose fathers had passed the period of maturity; but it is very large also, exceeding the insane among those convicted of assault and wounding (not represented in the Plate), and among sharpers. Sexual offenders have the largest proportion of mature fathers, the smallest of youthful fathers. Suspecting that among idiots a very large proportion of elderly fathers would be found, I applied to Dr. Langdon Down, who has kindly gone through the notes of one thousand cases, and confirmed this suspicion. He finds that in 23 per cent. cases there has been a disparity of age of more than ten years at the birth of the idiot child, the father in nearly every case being the elder, and that in many cases this disparity has reached more than 25 years. It appears, then, Dr. Down adds, that the disparity of age is a factor in the production of idiocy. It may be added that the elderly parent, by dying and leaving his children young and unprotected, has also a social influence in the creation of criminals. [Illustration: PLATE VII. RELATION OF AGE OF PARENTS IN NORMAL PERSONS, THE INSANE AND CRIMINAL.]