22 SCIENCE *PROGRESS 
tissues, is a problem too hypothetical for discussion here; it 
will suffice to say, that interference with the chemiotaxis of the 
cells of tumours of different kind would explain the phenomena 
thus far observed, their specific and their common features. 
Is the protection to be interpreted as due to some virus 
contained in the cancer cell? I think not. We have pointed 
out how mere inhibition of the connective tissue reaction would 
suffice to prevent growth, since its specificity indicates a delicacy 
of nutritive requirements on the part of the cancer cell far 
exceeding that of any other known biological reaction. We 
found that protection is conferred on the mouse only by mouse 
tumour, not by the preceding inoculation of the tumours of 
alien animals even so nearly related as the rat. One form of 
mouse tumour protects against another. We found that pro- 
tection is conferred on the mouse by the normal tissues of the 
mouse and not by those of strange species. There is thus 
something common to mouse tissue and mouse tumour which 
is not common to mouse tumour and the tumours of other 
animals. The protection cannot therefore be due to the presence 
in the tumour cells of a virus common to vertebrate cancer. 
Reviewing our experiments as a whole, I am of the opinion 
they establish that the cancer cell is really a cell of the mouse 
organism requiring the same food, the same kind of connective 
tissue and blood supply, to which it was accustomed in the 
mouse in which it developed. The only difference it exhibits 
is a qualitative one, viz. in the powers of growth and assimila- 
tion. Fortunately the cancer cell is very much at the mercy of 
subtle changes in its environment, and it is by taking advantage 
of this fact that we have been able to prevent its growth in 
the living mouse. 
The study of cancer in man became increasingly difficult in 
the course of time because of the mere multiplication of facts 
defying classification. Malignant new growths in man and in 
animals—even those of any particular organ and histogenetically 
related—exhibit an extraordinary variety in their histology and 
clinical behaviour. This variability is made yet more difficult 
to comprehend by the inconstancy in the behaviour of tumours 
histologically indistinguishable. Many attempts have been 
made to group the phenomena, e.g. by pushing the histogenetic 
subdivision of tumours to absurd lengths, by assuming that 
progressive loss of histological differentiation proceeds part 
