106 SCIENCE PROGRESS 



whole basis, objective and theoretical, of the cancer parasite 

 has been traversed again and again with the uniform conclusion 

 by those who have finished the journey that the cancer parasite 

 is the cancer cell." x One of the only consistent and highly 

 competent exceptions, as far as I know, is Borrel 2 ; since he 

 admits that the fact that cancer can be taken from one indi- 

 vidual and grafted upon another proves nothing in favour of 

 the parasitic theory ; it is difficult to see, however, why he still 

 adheres to the idea of a parasite. 



The motley throng which has in the past claimed the dis- 

 covery of the parasite of cancer consists mostly of the ignorant 

 but includes some very competent men. As has been frequently 

 pointed out, there are nearly as many different cancer parasites 

 as people who have claimed the discovery. Some claims are 

 so grotesque as not to be worth consideration, others have been 

 abandoned by their authors. It is probably not going too far 

 to state that, at the present time, no trained and competent 

 observer believes in any particular parasite except the one he 

 has himself discovered — which limits the supporters of parasites 

 to one man for each parasite. 



It is necessary here only to consider the general grounds 

 of disbelief in any specific micro-organism as the cause of cancer. 

 Of course, it is not possible to take a definite stand and say that 

 cancer cannot be due to an organism but that it can be so 

 caused is eminently improbable. 



As I shall show later, malignant growths may sometimes be 

 transferred from one individual to another by grafting small 

 portions of the tumour; but in no case will the tumour cells 

 survive in an animal of another species or even of another 

 variety of the same species. We know of no parasitic micro- 

 organism in mammals of which this is true. In the case of 

 " wheat rusts," one or two varieties of wheat may be susceptible 

 to a particular variety of rust but all other kinds of wheat are 

 immune to this particular variety of parasite. Thus parasite X 

 may thrive on variety A of wheat but wheat B may be naturally 

 immune. There is a way, however, by which X may be 

 rendered capable of attacking B. Parasite X is able to live 

 in another variety of wheat C ; if it be allowed to live for some 

 time on C, it is found to be capable subsequently of living 



1 James Ewing, Archives of Internal Medicine, vol. i. 1908. 



2 Borrel, Bull, de PInst. Pasteur, 1907, v. 497, 545, 593, 641. 



