1885.] Dr. Beck, Pathological Evolution, 35 



tions may succeed each other rapidly, and a consequent possibility 

 follows that surrounding circumstances may modify appreciably and 

 rapidly the constitution and organization of these germs, so that it is 

 possible for a germ, harmless now, to assume extremely virulent pro- 

 perties soon. 



That this is no mere speculation but actual fact, Pasteur has demon- 

 strated experimentally. His results are too well known to render it 

 necessary for me to do more than briefly indicate what they prove. 

 He experimented largely with the germs that are associated with 

 ''splenic fever" in sheep. This, an extremely infectious and fatal 

 disease, attacking sheep in various countries, Pasteur found was 

 caused by a germ which could be changed completely by simply 

 cultivating it in certain fluids. Starting with a virulent germ, he 

 found that after cultivating about 80 or 40 generations in such fluids, 

 he obtained a changed organism, unmistakeably derived from the 

 harmful parent, but quite incapable of exercising its functions, in 

 other words, quite incapable of causing the same virulent type of dis- 

 ease that the parent germ could. 



Here, actually in an artificial laboratory, it was possible to create 

 changes in an organism sufficiently tangible to be appreciable by 

 ordinary observation, and at least this fact then is proved : that the 

 germs we deal with in disease are not fixed in character, and that if 

 they could be experimentally modified, then in Nature's laboratories 

 existing around us far more exquisite modifications may be possible. 



The fields for speculation opened up by Pasteur's results are as wide 

 as they are beautiful, and I do not say too much when I declare that 

 they usher in a completely new epoch in the history and practice of 

 medicine. 



We have seen that ^e deal with living organisms of a low order, 

 demonstrated by Pasteur to be changeable in type. 



May we not go one step further, and speculate also as to their 

 changeability of species. Up to this I am perfectly aware this 

 question has neither been much discussed ner much entertained, and 

 it is j ust one of those speculations which does not admit of proof at 

 present. 



Because it has not been demonstrated that higher organisms have 

 ever changed their species, the conclusion at once is rushed at that 

 such a thing as modification of species is absurd. 



" DeQuatrefages," in an admirable book on the " Human Species," 

 published not long ago, devotes 500 or 600 pages to an attempt to 

 prove the unscience (if I, may so call it) of such an assumption, and 

 though expressing the greatest admiration for Darwin, he criticises 

 the assumption of " Origin of Species '^ in terms of the greatest severity. 

 He bases his antagonism to Darwin's hypothesis almost entirely upon 

 the assumption that no fertile hybrid has ever resulted from the 

 crossing of plants or animals of different species. Now, it is not for 

 me to say anything in this connection. I ^have, however, taken the 

 trouble to look into the matter, and I have the good fortune to be able 

 to quote from papers which appeared in the '' Botanische Jahrhilcher,''^ and 

 in the ''Naturforscher," by Herr W. 0. Focke, observations which at 

 least do not render this fact quite so certain as '*De Quatrefages " 

 would make out. 



Focke studied particularly the behaviour of different species of '' black- 

 berry " existing in Europe. Since 1857, i.e., prior to the publication 



