8 SCIENCE PROGRESS 
twenty sporadic mouse tumours, and conclusively proved that 
the cancer cells are merely transplanted into the new animals 
which provide them with nourishment, by means of a specific 
connective tissue reaction supplied afresh after each fresh 
transplantation. 
The criticism, that the tumours experimented with were not 
comparable with cancer in the human subject, was advanced as 
speedily as of yore, and with all the assurance born of a know- 
ledge of its effectiveness in discrediting earlier experiments. It 
is still feebly uttered by certain persons who lack extensive 
experience of cancer in animals. At last, however, it was to be 
met satisfactorily. Before proceeding to the study of other 
problems, Murray and myself spent much time in demon- 
strating, not only that the starting-point of the experiments 
has been cancer of the mamma, but were at great pains to 
reproduce artificially metastases, infiltrative and expansive 
growth—in short, all the main features of the disease in previ- 
ously healthy animals. With Cramer we drew a parallel 
between the growth of cancer under natural and experimental 
conditions. Haaland independently did the same for a number 
of carcinomata, and demonstrated that Ehrlich’s experimental 
sarcoma formed metastases, and behaved in other ways like a 
malignant new growth of the human subject. Apolant published 
an elaborate and valuable monograph on the histology and 
pathology of the mammary tumours of the mouse. 
These results are based on observations all but limited to the 
mouse. As yet our numerous inoculations of cancer in other 
vertebrates have met with only transitory success in rats and 
in a single carcinoma mamme of the dog. Even in the case of 
mice there are great difficulties to be overcome: for the other 
vertebrates many of the difficulties are still unsurmounted. The 
failures may be due largely to the relatively small number of 
inoculations made as contrasted with mice, as well as to 
difficulties associated with varying suitability of the animals 
employed, which the study of the transplantation of cancer in 
mice has proved is a factor of great importance. 
The inoculation of cancer in mice has been necessary on 
a scale perhaps unprecedented in the experimental study of 
disease, in order to prove that the cancer cell is able to grow 
and to proliferate continuously in animals of the same species 
only. To establish that the cancer cell is not transferred 
