smaller mammals is, on the other hand, sufficient to warrant reliance being placed upon them. Among the smaller mammals, none appear to offer material so favorable to the problem in question as do mice. Their short life cycle, rapidity of multiplication, convenient size and adaptability are supplemented by a varied and representative series of neoplasms serving to give them a unique value. Such studies as those of Tyzzer and Murray, and more recently those of Slye, Loeb and Lathrop, have proved beyond question that hereditary factors play an extremely important part in determining the incidence of cancer in mice. Though the fact of inheritance is undoubtedly established, the method of inheritance is as yet undetermined. Doctor Slye's work has repeatedly shown that non-cancerous parents may give cancerous offspring, and that cancerous parents may give non-cancerous offspring. This at once indicates a complicated type of inheritance, the exact nature of which is still in doubt.
Even in the case of inoculated tumors in mice, where our sole concern is growth of the tumor after its implantation, we have recently found that the interaction of many hereditary factors is involved (Little & Tyzzer, 1915).
Although smaller animals, as shown above, possess great advantages over man as material for studying the influence of heredity on the occurrence of spontaneous cancer, they, nevertheless, because of the fact that they are commonly genetically heterogenous, and because cancer is a disease influenced by sex and by the age of the animal, are not free from certain inherent limitations which make the analysis of the hereditary factors, beyond a certain point, extremely difficult. The only possible exception to this statement will be a race of animals so closely inbred as to be essentially homogeneous in its hereditary constitution. Various geneticists as, for example, Pearl and Jennings, have computed with accuracy the amount of inbreeding necessary before such a condition of homogeneity is closely approached. From their work it appears that many generations of continuous closest inbreeding, such as, for example, own brother and sister matings, must be made before homogeneity of genetic constitution is approximated. It is safe to say that none of the material thus far employed in investigations with spontaneous tumors meets this requirement.
Some doubt has been expressed as to the applicability of the results obtained with small mammals such as mice, for example, to the problem of human cancer. In so far as these results may affect the acceptance of the fact of heredity, doubts are not justified by analogy with any of the cases of inheritable characters, so far investigated. It is possible and permissible to argue the existence of hereditary tendencies to cancer in man on the basis of proved existence of such tendencies in other mammals. Similar arguments have been shown to apply to albinism, spotting, shape of hair, color of eyes, certain abnormalities in growth of the bones, and to many other characters.
