RELATION OF HEREDITY TO CANCER 201
division have their origin. It is also certain that hereditary differences in the nature of the cell material among animals of a single species exist These differences will naturally be an important factor in the reaction of such material to a given stimulus of the internal environ- ment. For example, we may imagine that a certain type of internal environment may cause the material within the connective tissue of individual (a) to show no abnormality of gro^vth, while the material forming the connective tissue of individual (6) of the very same species may be inherently different to a point where an identical in- ternal environment will start up uncontrolled growth.
On the other hand, two individuals may have connective tissue which is similar in respect to its reactions to a certain stimulus of given internal environment X, but may differ in their internal environ- ments because of differences in amount or exact chemical nature of internal secretions or other important agents. In one animal, con- nective tissue Y might show no effects of internal environment X, while in the other the interrelation of connective tissue Y with environment X' might lead to uncontrolled growth.
This rather lengthy treatment of the subject of internal environ- ment has for an object to emphasize the extremely complicated biolog- ical nature of cancer. Occurring as it does, usually in middle or old age, it is at a point most completely removed in time and space from the carriers of the elementary hereditary tendencies — the germ cells. In snch an animal as man, where the average age for the appearance of cancer, broadly speaking, is about forty-five to fifty years, the oppor- tunities for the effects of the internal environment to become excessively amplified and complicated are, of course, obvious. Injury as well as inflammation of long duration, long recognized as probable agents in the initiation of uncontrolled or abnormal growth, are also much more hkely to be of importance in a very slow-growing mammal such as man, than in a rapidly growing mammal like the mouse. This follows from the fact that the critical periods in internal environmental changes in man are in themselves far longer in duration than they are in mice, and an injury or irritation, therefore, has more chance of occurring in one of these periods. In the cases of irritation or injury the inherited nature of the individual is of prime importance. A great number of men may use tobacco to an equal extent and yet only part of them may develop cancer of the buccal cavity. In such cases the irritating stim- ulus may be equal, but the nature of the reaction of the individual's tissue may differ widely.
Again and again we are driven back to the ground that the nature of the mouse, or of the man, by which we mean the nature of his hered- itary living material, determines his physiological reactions to any given environment, and further we may add that it determines to a VOL. ni — 14.
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