10 THE SCOPE OF THE ENQUIRY [CH. i 



A CONSIDERATION OF THE POSSIBILITIES. 



The various possibilities to be considered are five in 

 number. It will be seen that the first three are instances of 

 variation and the remaining two, instances of transmutation. 



1. Simple variation. Modifications may occur in the 

 characters of an organism, involving either the loss of some 

 feature previously regarded as characteristic or the acquisition 

 of some other feature not hitherto considered to be so, such 

 modifications not being so numerous, however, or so funda- 

 mental, as to lead to any doubt as to the proper identification 

 of the organism. For example, Twort (1907) succeeded, in the 

 course of two years, in training a strain of B. typJiosus to 

 ferment lactose, and both Twort (1907) and Penfold (1910 A) 

 produced pure strains capable of fermenting dulcite, which 

 the usual variety of typhoid bacillus is practically unable to 

 do. These new strains retained qualitatively all the other 

 properties of the B. typhosus unchanged. Similarly Miss 

 Peckham (1897) induced indol formation in numerous strains 

 of B. typhosus. 



2. Variations in different directions associated. The 

 acquirement of some fresh character may be associated with 

 the loss simultaneously of some other character previously 

 possessed or, with the acquisition of a second new character, 

 and in some cases this association may prove to be invariable 

 under the same modifying conditions (vide p. 146). For 

 example, Eyre and Washbouru (1899) observed that a non- 

 virulent strain of the pneumococcus growing readily at 20 C. 

 could by "passage" be converted into a highly virulent strain 

 which was then unable to grow at a temperature below 37 C. 

 The reverse change showed the same relation between the 

 virulence of the organism and the temperature at which it 

 would grow. 



Jenner (1898) was able to revert B. coli capsulatiis to an 

 unencapsulated form by cultural methods and found that the 

 new variety had lost the power to coagulate milk, and instead 

 of being highly pathogenic to white mice had become much 

 less so or even non-pathogenic. 



