MUTATION: CONSTANCY OF TYPES 35 



by Gaffky three decades ago. It appears to be reasonably certain 

 from what is known of bacteria and the manifestations of disease 

 they induce that mutation is an infrequent phenomenon: attenuation 

 and the partial suppresion of characteristics, on the contrary, appear 

 to be quite common. The available evidence indicates that bacterial 

 types are stabile under natural conditions : there is no definite evidence 

 in favor of the view that bacteria change slowly or abruptly either in 

 their morphology or in the changes they induce in their environment 

 in the sense that entirely new, unrelated types are developed de now 

 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 definitely established. 



