Suppose again that all old men died of pneumonia. Could we say that susceptibility to pneumonia, after eighty, is inherited? Again, yes! But again we could get no information as to the way in which this susceptibility is inherited without crossing to an immune race. Now suppose there are strains of mice all of which die of cancer after their first year. Could we say that in them cancer is inherited? The answer would depend in part on what connotations the word _inherit_ carries with it, for, either susceptibility might be meant, or the “spontaneous” development of cancer might be meant. The latter interpretation is, I think, generally implied, which carries with it two further implications. First implication, viz., that when a certain age is reached, a certain inherited complex leads to the development of cancer in one or more regions of the body. Here some such process as that of the hardening of the arteries seems to be vaguely implied. Second implication, viz., that a change in method of growth (a release from the ordinary restraining influences) suddenly occurs, beginning in a single cell of some particular tissue. Stated in this second way, the appearance of spontaneous cancer suggests at once a comparison with the mutation process that is known to occur in somatic cells as well as in germ cells. Now if the first interpretation is to be placed on the word heredity, when applied to cancer, there is nothing more to be said, except that the only way such a situation can be studied as a genetic problem is to out-cross the strain of cancer mice in question to another that never develops spontaneous cancer. But if the second interpretation is implied, then the whole situation is put in a very different light. Let us examine this a little more closely.