142 SUMMARY [OH. x 



This aspect of the subject has been considered at length in 

 the sections dealing with Fermenting Power and Virulence 

 (Chapters Y and VI). 



Since we have no absolute criterion as to what constitutes 

 a "species" amongst bacteria, dissimilarity in the several 

 characters they present is our sole guide to classification. 

 In other words the distinction between a "variety" and a 

 "species" depends simply on less or greater divergence in 

 character. The difference, therefore, between variation and 

 transmutation is one of degree only ; or, looking at the matter 

 from another standpoint, we may say that the same degree 

 of deviation from type may be interpreted in one case as 

 variation and in another as transmutation. This will be readily 

 understood if it be borne in mind that the various types or 

 "species" of bacteria which we are able to distinguish have 

 developed from a common stock. In the case of some of them 

 the differentiation dates from a remote past and the specific 

 characters are comparatively fixed. In the case of others 

 differentiation is of more recent date and the newly acquired 

 characters are less permanent and "reversion" in one or other 

 character is more frequent. In yet a third class the groups 

 of closely allied organisms the gradual process of differentia- 

 tion is only now taking place and it is not yet clear which 

 characters are of specific value. During the process of evolu- 

 tion, in all its stages, there is a tendency shown on the part 

 of the organism to revert towards the original type. Such 

 reversion in one or more characters, although of no greater 

 significance in the case of one class or another of the three 

 we have described, is likely to be differently interpreted. If 

 differentiation is well advanced, a partial reversion in character 

 will merely present itself as an unimportant variation. If 

 differentiation has not progressed very far, a reversion no 

 greater in degree may confuse the identity of the organism 

 concerned sufficiently to suggest the possibility that trans- 

 mutation has occurred. If the process of differentiation is 

 still incomplete, a reversion in character even smaller in degree 

 may entirely obliterate the faint lines of division that we have 

 been able to trace out. The error of assuming too hastily that 



