CH. X] SUMMARY 151 



gelatin, producing a uniform turbidity in broth, and it retained 

 its vitality for many months ; it also exhibited diflferences in 

 its morphology, "instead of isolated diplococci and strepto- 

 cocci, large masses of cocci and diplococci were found, and 

 forms dividing into tetrads were common." Nevertheless this 

 avirulent saprophytic pneumococcus could, by a single " pas- 

 sage " through a rabbit, be converted into a typical parasitic 

 pneumococcus of high virulence. The occurrence of such a 

 remarkable transition would be regarded as more significant 

 if it were not that both organisms bear the same name and 

 are considered — in spite of the many differences existing be- 

 tween them — to be variants of each other. 



If we consider the possibility of a similar transition in the 

 case of two races of bacteria less closely associated with each 

 other, we find little direct evidence in proof of its occurrence 

 — and this often of doubtful value — but a great deal of cir- 

 cumstantial evidence in favour of the supposition that it may 

 occur. We have discussed at length (Chapter VIII) such a 

 possibility in the case of organisms found in close association 

 in the body, such as Hofinann's baciUus and the Klebs-LoefHer 

 haciWus, Staphylococctis epidermidis and Staphylococcus pyo- 

 genes, Micrococcus catarrhalis and the meningococcus, and 

 others. 



Finally, we have discussed in detail (Chapter IX) the re- 

 cords of certain experiments in the course of which bacteria 

 became so changed in character as to suggest that they had 

 undergone transmutation. 



In the first series of experiments — those of Major Hor- 

 rocks — the results seem to be capable of explanation on other 

 grounds. In the first place adequate precautions do not appear 

 to have been taken to guarantee the purity of the strains at 

 different stages of the experiment. In the second place, many 

 of the changes in character stated to have been observed may 

 be regarded as examples of temporary variation only, similar 

 to those recorded by many other observers. 



The second series of experiments — those of Schmitt, and 

 of Mlihlens, Dahm and FUrst, and of the writer — which 

 suggest the occurrence of transmutation between different 



