THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 279 



produced. Similar persistent modifications have also been obtained in 

 Drosophila by heat.^ A culture heated for some hours to just below the 

 lethal temperature (i.e. to about 37° C.) shows three types of effects: 

 (i) a rise of the gene-mutation rate (p. 381); (2) the production of 

 non-hereditable abnormaUties which often closely parallel gene effects 

 (p. 191); and (3) persistent modifications which are inherited through 

 the female for a few generations and then gradually return to normal. 

 In Drosophila further treatment does not increase the degree of 

 modification. 



Some authors have been tempted to suggest that persistent modifica- 

 tions may play an important part in evolution by bringing about an 

 adaptation to the environment. The experimentally produced modi- 

 fications are, however, not adaptive in nature; they seem to be caused 

 by a sHght alteration to the cytoplasm but there is no reason why this 

 alteration should lead to the production of organisms which are adapted 

 to the inducing conditions. Moreover, a persistent modification of the 

 cytoplasm could only be important for evolution if it caused the genes 

 to become altered in conformity with it^ and, at least over the fairly 

 short periods for which experimental evidence is available, there is no 

 evidence that this is possible. 



Certain phenomena of variation as it occurs in nature have been 

 discussed in terms of persistent modifications. Woltereck^ has shown 

 that in Cladocera some species have many local forms in different 

 regions, while others are much more constant. The variable species are 

 those inhabiting lakes in northern Europe and America, v/hich they can 

 only have reached comparatively recently, after the last Ice Age, say 

 ten thousand years ago. Their variations are strongly inherited and look 

 as though they were adaptive; the pelagic types, for instance, occur 

 only in lakes with deep water with a thermocline (a sudden change of 

 temperature at a certain depth). Daphnia cucullata from L. Frederiks- 

 borg in Deimiark (3-4 metres deep with no thermocline) were trans- 

 ferred to L. Nemi in Italy (34 metres deep, thermocline). They acquired 

 a pelagic form. After fourteen years they were taken back into the 

 laboratory and in about forty generations they had been brought back 

 to the original Frederiksborg type. Thus the modification which they 

 acquired in the L. Nemi conditions was persistent but not fixed. 

 Another race, from L. Esrom in Denmark, which was in appearance 

 very like the L. Nemi form, could not be converted in the laboratory 



^ JoUos 1934. For criticism, see Timofeeff-Ressovsky 1937. 

 2 Woltereck 1932, 1934. 



