THE MICROBES OF HUMAN DISEASES. 217 
sources. The former multiplied rapidly and energeti- 
cally, sueceeding each other up to the tenth generation, 
while those from Philadelphia only went to the fourth 
or fifth generation, and those taken from the tongue 
did not go beyond the third. It must be observed that 
the diphtheritic angina of Philadelphia is much less 
fatal than croup, and the first attempts at inoculation 
made by Formad and Wood produced doubtful results, 
precisely because they were made with the microbe 
of diphtheritic angina, which is an attenuated form 
of the microbe of croup. The organism is the same, 
but it is modified by the medium in which it is 
developed, and the vitality of artificial cultures is in 
direct proportion to the malignity of the disease from 
which the germs for sowings are derived. 
The following theory may be deduced from these 
facts, which will explain all cases of diphtheria :—A 
child contracts a simple catarrhal angina, or laryngitis ; 
the micrococci, which up to this time remained inert 
in the mouth, begin to grow and multiply under the 
influence of the inflammatory products which favour 
their development ; the plant which has been dormant 
becomes widely diffused. There are many degrees 
between croup with malignant complications and the 
mildest form of diphtheritic angina, as all practical 
‘physicians know. More or less numerous germs of 
micrococci float in the air, or—which appeared to be 
the case at Ludington—are conveyed in drinking- 
water, and they may encounter more or less favourable 
