MUTATION: CONSTANCY OF TYPES ■ 35 
Exposure to unfaAorable environmental conditions may also suppress 
important characters: Pasteur's celebrated experiment of growing 
anthrax bacilli at 43° C. for some hours and establishing a sporeless 
variety is a familiar example. The suppression of characters as out- 
lined above is frequently important as the starting point for new 
adjustments between pathogenic bacteria and their hosts. On the 
other hand, Klienberger^ has apparently been successful in developing 
the power to ferment salicin in a strain of B. coli by repeated transfers 
through mediums containing this glucoside. The attempt was 
unsuccessful with other carbohydrates, and the strain gradually 
reverted to type when it was cultivated without access to the salicin 
for some time. 
Turning to the production of disease in man, it is certain that at least 
some organisms produce the same reaction today they did years ago: 
tuberculosis appears to be the same disease today it was centuries 
ago, as is evidenced by the lesions found in Egyptian mummies."^ 
Clinically, the observations of Hippocrates would be a fair exposition 
of the phenomena seen in tuberculous patients at the present time. 
Leprosy also appears to be the same entity now it was during the 
middle ages, although the geographical distribution is much more 
restricted. With respect to more acute diseases, which require more 
careful examination to differentiate them, the evidence is less certain, 
although typhoid bacilli do not appear to have changed since they 
were first isolated by Gaffky nearly five decades ago. However, it 
seems probable that lethargic encephalitis, and possibly poliomyelitis 
are diseases of comparatively recent origin. It appears to be reason- 
ably certain from what is known of bacteria and the manifestations of 
disease the}- induce that mutation is an infrequent phenomenon: 
attenuation and the partial suppression of characteristics, on the con- 
trary, appear to be quite common. The available evidence indicates 
that bacterial types are relatively stable under natural conditions: 
there is little definite e\idence in favor of the view that bacteria fluctu- 
ate abruptly either in their morphology or in the changes they induce 
in their en\ironment in the sense that entirely new, unrelated types 
are developed de novo from preexisting types. This does not preclude 
the possibility that such changes have taken place in the past; rather 
that such changes, if they have taken place, have not been satisfactorily 
established. 
1 Centralbl. f. Bakt., I Abt. orig., 1927, 101, 462. 
^ Smith and Ruffer: Pottsche Krankheit an einer aegyptisohen Mumic, Zur histori- 
schen Biol. d. Krankheitserreger, No. 3. Ruffer: Cairo Sci. .Jour., 1910, .4, No. 40. 
