CAUSES OF GENETIC VARIATION 215 



properties be selected as the diagnostic characters. There are 

 many types perfectly distinct and others which intergrade. 

 Some of the types change greatly with conditions while others do 

 not. This is exactly what we encounter whenever we study the 

 problem of species on an extended scale among the higher forms 

 of life. 



There is now practically complete agreement among bac- 

 teriologists that the observations made first by Massini on the 

 change in color of Bacterium coli mutabile grown in Endo's 

 medium, associated with the acquisition of the power to ferment 

 lactose, are perfectly reliable and free from possibilities of mis- 

 take. The work has been extended and confirmed by many 

 workers, especially R. Miiller, who finds that this bacterium can 

 similarly acquire and maintain the power to ferment other 

 sugars. A careful account of the whole subject written by Miiller 

 for the information of biologists will be found in Zts.fiir Abstam- 

 mungsl., VIII, 1912. After discussing the biological significance 

 of the facts, he concludes with a caution to the effect that bacteria 

 are so different from all other living things that generalizations 

 from their behavior must not be indiscriminately applied to 

 animals and plants. 



In all work with this class of material there is obviously 

 danger of error through foreign infection of the cultures, but 

 there can be no doubt that though some of the "mutations" 

 recorded may be due to this cause, the majority of the instances 

 observed under stringent conditions are genuine. 



Another and equally serious difficulty besetting work with 

 bacteria and fungi cultivated from spores is that the appearance 

 of variation may in reality be due to the selection of a special 

 strain previously living masked among other strains. This 

 possibility must be remembered especially in those instances 

 which are claimed as exemplifying the effects of acclimatisation. 

 Manifestly this consideration can be urged with most force when 

 the strain which gave rise to the novelty was not raised from a 

 single individual spore. Moreover, when once the possibility of 

 spontaneous variation is admitted, it must be difficult to be quite 

 confident that any given variation observed is in reality due to 



