MODIFICATION OF FUNCTION. ' 19 



permanent, and it has been held by some that one species of micro- 

 organism may, under appropriate conditions, change its nature com- 

 pletely, and appear in the guise of another species separated from its 

 original form by a wide gap, and this whether the evolution be 

 viewed morphologically or functionally. Such was the doctrine pro- 

 mulgated by V. Naegeli, and still maintained by some of his disciples. 

 But the results of more recent research on this point have rendered 

 such a position apparently untenable, and it has been now pretty 

 generally abandoned as the direct outcome of imperfect methods of 

 research employed in the elucidation of a very difficult, and till recent 

 years a very obscure subject. 



Modification of Function. 



17. It is, however, by no means inconceivable that in favourable 

 circumstances a micro-organism may change its functional, and even, 

 within certain closely circumscribed limits, its morphological charac- 

 teristics, in consonance with what is known of the modifications 

 produced by domestication or similarly varied conditions amongst 

 the higher animals ; but as in the latter, so also in the former, the 

 thoroughly accredited law of reversion to type comes into play so 

 soon as the abnormal circumstances which occasioned the devia- 

 tion are removed, and the species once more assumes its typical 

 characteristics. 



For this reason, the statements of Buchner and some others have 

 been viewed with grave doubt. They claim to have effected through 

 a long series of gradations a complete change from one well defined 

 species into another, equally well defined, with functions quite dis- 

 tinct from, but morphologically closely allied to, the initial species. 

 They effected this transformation by delicately graduated modifications 

 in the surroundings of the initial species, and, when completed, they 

 regard the change as permanent. At this point, however, it must be 

 distinctly recollected that the long chain of altered conditions thus 

 employed has been in every link exposed to extreme dangers, which 

 even consummate skill as regards technique cannot render nugatory ; 

 for in every one of the hundreds of re-inoculations carried out in pro- 

 ducing the vast number of so-called " generations," between the 



