THEORIES AND PROBLEMS OF CANCER 107 



on A. C is called the bridging species. Somewhat similar 

 attempts have been made in the case of cancer. Growths 

 originating in one breed of mice have been transferred with 

 difficulty from race to race (e.g. English, French, German, Danish) 

 but never survived when subsequently introduced into rats for 

 a longer period than it did before it had been passed through 

 two or more different races of mice. Consequently, if cancer be 

 caused by a parasite, there must be a different parasite for every 

 different kind of animal that suffers from cancer ; none of the 

 parasites must be able to survive in any species or variety of 

 animal except the one to which it belongs : yet all these different 

 parasites produce precisely the same results in the different 

 kinds of animals. All the parasites which we know to be 

 capable of causing the same disease in different kinds of 

 mammals are able to survive in a number of different species. 



But this after all is one of the lesser difficulties in accepting 

 a parasite as the cause of cancer. Many of the points involved 

 in some of these difficulties are so technical that short of writing 

 a treatise on the general pathology of tumours, it would be impos- 

 sible to make them clear to the general reader. One or two 

 of the most striking examples must suffice. 



Having gained an entrance to the system, though in some 

 cases parasitic micro-organisms may remain more or less 

 localised, when they extend their ravages upon their host to 

 different parts of the body they produce similar changes in 

 the cells and similar results whatever may be the tissue they 

 attack. Some parasites show a preference for particular parts 

 of the body or particular kinds of tissue, others do not. Malig- 

 nant growths occur in every tissue in the body with but few 

 exceptions, such as nervous tissue. When, however, a metas- 

 tasis, that is a secondary tumour or extension of the disease 

 to another part of the body, occurs in a person suffering from 

 cancer, this metastasis consists of cells similar in character to 

 the original or primary growth : it therefore must be supposed 

 that when the parasite gains entrance to the body of an animal, 

 it takes on a new power which enables it, when it passes to 

 another part of the body of its host, to transform the cells 

 of this other part and give them the characters of the 

 cells among which it lived at first in the body of this par- 

 ticular host. The only other alternative is to believe that 

 besides a different species of cancer parasite existing for 



