
304 THE EVOLUTION OF LIFE 
fever patients infected their friends with diphtheria, 
and diphtheria patients in their turn infected their 
friends with scarlet fever.” He is so positive as 
to what he has observed, that he speaks of the 
transformation as ‘‘an established fact.” 
Of course, if this is true in regard to the genera- 
tion of diphtheria from common sore-throats under 
certain conditions; and if a diphtheria patient may 
at times give rise to contagia which set up scarlet 
fever in another, we have practically a de novo origin 
for this latter disease also;! and in each case it can 
only be supposed that minute transitions in the 
activities or properties of the micro-organisms con- 
cerned must have occurred; so that so-called common 
or non-pathogenic organisms in the throat have been. 
converted into pathogenic forms, and that, among 
these, under special conditions, other variations have 
been brought about, leading to an apparent trans- 
formation of one disease into another. 
Lehmann and Neumann again, in their “ Prin- 
ciples of Bacteriology,” call special attention to the 
probable mode of origin of that fearful scourge Pest, 
more commonly called Plague. They cite it as 
affording an important example of that transforma- 
tion of non-pathogenic into pathogenic organisms, 
which we are now considering, and in which they 
also are firm believers, -They say (p. 217)) "sin 
India, Hankin and Yersin have many times culti- 
1 From time to time cases of Scarlet Fever have been reported in 
which it is almost impossible to believe that they have not been 
engendered de novo. An extremely well-authenticated case of this 
kind has recently been recorded by Dr R. J. Mackeown of H.M.S. 
Alacrity (see Brit. Med. Journ., June 11, 1904, p. 1370). 
