106 ON VARIABILITY AND ADAPTATION 



limited number of cases a harmless saprophyte is capable of 

 developing into a pathogenic microbe, at the present moment 1 

 the facts are wanting to support this hypothesis. The extreme 

 conclusion to be reached now is that there are species of patho- 

 genic micro-organisms whose virulence is capable of manifesting 

 great modification, and that such modification is fitted to explain 

 some of those differences in the course of zymotic disease which 

 must have arrested the attention of all medical men. 



On ceetain Characteristics of the Bacteria as a Whole 

 affecting the liability to variation 



At the very onset we should expect to find the bacteria 

 peculiarly well adapted for the study of the most elementary 

 problems of evolution : their organization, as compared with 

 other forms of fife, is so simple, the generations — if generations 

 they may be called 2 — succeed each other with such extraordinary 

 rapidity, and, what is of yet greater importance, our study of 

 these forms would seem to be wholly freed from the perturbing 

 influence of sex. On the one hand there can, in the bacteria, 

 be no tendency towards the production of races through sexual 

 agency ; on the other the reverse tendency is equally absent, 

 namely, the tendency of conjugation to preserve the mean, to 

 lead to the development of individuals approximating to the 

 type. 



Here, then, in studying these unicellular asexual bacteria, 

 we investigate the simplest and most fundamental principles of 

 heredity and evolution, and the influence of environment be- 

 comes most evident. Weismann, who in the higher forms of 

 life would make the development of individual differences 

 and the inheritance of the same almost wholly dependent upon 

 the phenomena which accompany the fusion of the male and 

 female reproductive cells, freely acknowledges that to the 



1 [i.e. in 1892.] 



2 The word generation as applied to the Schizomyeetes is not a little un- 

 satisfactory — yet I know no other word at present in use that is capable of 

 replacing it. Apart from the sexual significance that may attach to it, this 

 word embraces the idea of parentage. Now, among the bacteria there is 

 no true parentage, for what happens in the process of fission is that the bacillus 

 divides into two portions, each of which becomes a separate individual, but 

 neither can be regarded as the parent form ; each equally with the other 

 represents, and, to a certain extent, is, the earlier single individual. 



