20 SCIENCE ‘PROGRESS 
other growths. There is thus a degree of protection which is 
common, and a certain degree which is specific. The spontaneous 
disappearance of tumours occurs, entirely beyond our control as 
yet. But the protection it confers can be imitated artificially. 
The same degree of common protection can be conferred by the 
preceding inoculation of tumour material if followed by no 
growth, ze. if absorbed, and this is not so remarkable as the fact 
that it can be induced also by the injection of the normal tissues 
of the mouse and most readily by normal blood; but not by the 
cancers or tissues of alien species. The far-reaching signifi- 
cance of the protection which can be induced by carcinomatous, 
by sarcomatous, or by normal tissues, causes me to emphasise 
that our observations and Ehrlich’s are in essential agreement. 
We require to learn whether the protection thus conferred is 
actively induced or only passively conferred. In the case of 
blood this question was most easily settled. The protection is 
not passively conferred by the serum, but is actively induced by 
the blood cells. To sum up, we are able to so modify the soil 
provided by mice that cancer cannot growin it. For indications 
of the change we are dependent on the behaviour of the living 
cancer cell in the living mouse; we are dependent on experiment 
absolutely. 
Jensen recorded the disappearance of tumours in 1902-3. 
The first occasion on which we noticed a large transplanted 
tumour disappear was while studying a batch of mice with his 
tumour in August 1904. This tumour was exhaustively studied, 
and the important part phagocytosis played in the process 
discovered, not only in the case of this entire tumour, but in 
more limited areas of a very large number of tumours, We 
have since recorded the same process for other carcinomata, and 
its importance has been emphasised by others, especially by 
Clowes. We have observed the same process in localised areas 
of spontaneous tumours. There can therefore be no doubt that 
the animal primarily attacked attempts to protect itself by the 
same means as a normal animal after successful inoculation. 
The investigation of the nature of protection is bound up 
with very great difficulties, which are not diminished by the 
absence of any analogy with what we know of immunity to 
infective diseases. Up to the present I have spoken of the 
propagation of the tumours as if it meant a mere culture 7m vivo 
of the cancer cells. This, however, is only part of the truth. 
