Tar does not have this effect upon other viruses which do not cause proliferation of cells. Even more striking is the effect of tar upon the papilloma virus affecting rabbits. In this disease small warts appear on the rabbit’s ears, and these warts may occasionally develop into malignant growths, but only after many months. An experiment with papilloma was conducted wherein one ear of the rabbit was tarred while the other was left untreated. The untreated ear of the rabbit was then injected with the papilloma virus. After a short time the result showed that the virus was producing proliferating growths on the tarred ear became malignant almost from the beginning. This did not happen on the ears of rabbits which were not tarred. Another case which is more interesting, concerns the results of injecting tar into the muscles of perfectly normal fowls. Not only did a tumor appear to result in a number of cases, but it was possible to pass on such a tumor to other fowls by injecting them with a filtrate of the first tumor. Apparently the virus had risen or come to light. We do not know exactly what the relationship between tar and virus is. We can only suppose that the tar acts in some way to upset equilibrium between the cell and a virus which it contains. As to the case of the rabbit, we knew that a virus was present. In the case of the fowl we must assume the presence of a latent virus which was stimulated into action by the effect of the tar on the cells. On the other hand in the case of swine influenza, we have already seen an example of how an external stimulus was necessary to cause a latent virus to flare up and produce symptoms. It was necessary for an injection of bacteria, dead or alive, to be made into the pig before disease symptoms developed. Mention was made about the latent viruses in plants. However, there is no exact parallel in them to the Rous sarcoma. One virus which affects sugar-cane does give rise to elongated swellings or galls. These so called swellings or galls extend along the larger veins or vascular bundles of the leaves and are, in fact, formed by the abnormal growth of the tissues comprising these bundles. These abnormal tissues can be seen well under the microscope using high power microscopy. If a diseased cane is split open, more of these galls can be seen in the vascular bundles of the stem when examined under a microscope using high power microscopy. Abnormal growths of another kind occur in two or three other plant virus diseases. Certain viruses which affect the tobacco plant give rise to peculiar outgrowths on the under-surfaces of the leaves. These outgrowths vary from mere thickenings of the veins to quite large leafy proliferations. Such outgrowths might perhaps be likened to the swellings produced by certain viruses on the skin and ears of rabbits. To better study these different viruses affecting both path and animals, it is best to view them with them under the microscope using high power microscopy.

Some strains of mice used in cancer research have an incidence of breast cancer in breeding females of 80 to 90 per cent. In others the incidence is almost nil. In hybrids between two such strains it has been found that the incidence of breast cancer in the offspring depends not upon the ordinary rules of inheritance but wholly upon whether the mother came from a high or low cancer family. The mice born from high cancer mothers developed breast cancer in their later life. Research results found that mice from the low cancer strain would develop a high proportion of cancer if they were suckled from birth by high cancer-strain mothers. Conversely, mice from high cancer mothers would have quite a low tumor incidence if they were fostered by mothers of the low cancer strain. It appeared as if something affecting the occurrence of cancer in later life was transferred to the young in the mother’s milk. If young mice of the low cancer stock were suckled by high cancer mothers and consequently developed cancer later despite their hitherto unstained family escutcheon, then, on inbreeding them, their children and grandchildren were also very liable to breast cancer; the blot on the escutcheon was passed on. On the other hand, the young of the high cancer stock which escaped the disease by being fostered by low cancer mothers were able to pass on their freedom from the dreaded cancer to their descendants.

These facts are compatible with the idea of a virus transmissible in the mother’s milk. Such a virus might for a time, at any rate, remain quite latent. Ultimately some stimulus, or stimuli, acting over a long period, might cause the virus to assume an active part and so give rise to the breast carcinoma.

Even if this theory of the latent virus is correct, it does not follow that the virus is the only thing concerned in the cause of cancer. There are probably several conditions which must be fulfilled. First, there must be the latent virus lying in wait. Then there must presumably be certain hereditary characters or tendencies. Lastly, there are other factors necessary, which tip up the balance in favor of the tumor. These may be some external stimulus like a carcinogen, or a hormone as in breast cancer.