140 CONTEMPORARY SCIENCE 



ing in mind Behring's dictum that to produce a thera- 

 peutic serum, it is essential to immunize highly susceptible 

 animals, it becomes evident why success has not crowned 

 the many undertakings to prepare an antipoliomyelitis 

 serum in the horse or other large animal, and why it is 

 only by the use of swine themselves that an anti-hog- 

 cholera serum has been secured. 



The investigation of this class of excessively minute or 

 filterable parasites casts a sharp ray of light into a neigh- 

 boring field of biological research which at the time 

 aroused hopes of further rapid progress but which the 

 intervening time and effort have not realized. 



Perhaps no subject in experimental pathology has been 

 pursued with more thought and energy than the one to 

 which the name of cancer research is applied. The rea- 

 sons are obvious. The nature of the source of the can- 

 cerous tumors is still shrouded in essential darkness. It 

 is, of course, known that cancer sometimes follows upon 

 prolonged irritation and inflammation of tissues variously 

 excited. But what the immediate impulse is that calls 

 forth the cancerous state is unknown. And yet advances 

 have come from the study of the spontaneous and trans- 

 plantable cancers in mice, rats and some other animals. A 

 long series of biological conditions governing the growth 

 and recession of the tumors have been uncovered, and by 

 altering those conditions, on the one hand growth can 

 be promoted, and on the other, retarded. In this way, 

 Murphy and his co-workers have accounted for the in- 

 fluence of the action of the X-ray in affecting cancer 

 growth ; and by observing the correlative effects on the 

 lymphoid structures of the body, which are very sensitive 

 to the rays, and the changes corresponding to them in the 

 circulating blood, they have so altered the scale as almost 

 at will either to abotish or stimulate the development of 

 mouse cancers. 



But these experimental results and others of a class in 

 which the defensive forces of the body can be marshalled 



