Little is known in regard to the influence of inheritance on the formation of tumors.  Study of the tumors of mice show a slightly greater susceptibility to tumor formation in the progeny of mice who have developed tumors.  Studies of human families seem to show that heredity has a slight influence, but in the frequency of tumors such statistical evidence is of little value.  The question of inheritance has much bearing on the origin of tumors.  If the tumor is accidental and due entirely to extraneous causes, inheritance is not probable; but if there is some predisposition to tumor formation in certain individuals due to some peculiarity, then inheritance may exert an influence.

The question as to whether tumors are an increasing cause of disease is equally difficult of solution.  The mortality statistics, if taken at their face value, show an enormous increase in frequency; but there are many factors which must be considered and which render the decision difficult and doubtful.  Tumors are largely a prerogative of age, and the increased duration of life which preventive medicine has brought about brings more people into the age when tumors are more common.  Owing to the greater skill in the diagnosis of tumors, especially those of the internal organs, they are now recognized more frequently and more deaths are correctly ascribed to them.  Deaths from tumors were formerly often purposely concealed and attributed to some other cause.

No age is immune to tumors.  They may be present at birth or develop shortly afterwards.  The age from five to twenty years is the most free from them, that from forty-five to sixty-five the most susceptible, particularly to the more malignant forms.

A tumor is a local disease.  The growing tissue of the tumor is the disease, and it is evident that if the entire tumor were removed the disease would be cured.  This is the end sought by surgical interference, but notwithstanding seemingly thorough removal, the tumor often reappears after an interval of months or years.  There are many conditions which may render the complete removal of a tumor difficult or impossible.  It is often impossible to ascertain just how far the tumor cells have invaded the neighboring structures; the situation of the tumor may be such that an extended removal would injure organs which are essential for life, or at the time of removal the tumor cells may have been conveyed elsewhere by the blood or lymphatic vessels.

Successful removal depends mainly upon the length of time the tumor has been growing.  At an early stage even the most malignant tumor may be successfully removed.  It is evident from this how disastrous may be the neglect of proper surgical treatment of a tumor.  The time may be very short between the first evidence of the presence of a tumor and the development of a condition which would render complete removal impossible.