THE ORIGIN OF VARIATION 39 



not surprising that the percentage of immunity eventually 

 rose. 



xv. Agar (191 3). Effect of temperature and medium on 

 Simocephalus. 



Agar succeeded in inducing heritable changes in the size 

 of Simocephalus vetulus (Cladocera) by raising the temperature 

 of his cultures. He also experimentally induced an outward 

 flanging of the edges of the carapace by keeping his cultures 

 in Klebs' solution. These modifications were reproduced in 

 F x individuals, the mothers of which had been restored to 

 normal conditions just before the eggs were laid, and per- 

 sisted for some generations, though they became progressively 

 modified, i.e. they behaved as ' Dauermodifikationen.' Agar 

 interprets them as effects of ' parallel ' modification. As 

 reproduction was parthenogenetic, inheritance may have been 

 through the cytoplasm. 



xvi. Woltereck (1908, 191 1, 1921, 1928). Modification of 

 the ' helm ' in Daphnia. 



The work of Woltereck on the modification of the ' helm ' 

 of Daphnia stands in a rather different category from the 

 work just described. Woltereck claimed to have induced a 

 temporarily heritable change in the form of the ' helm ' 

 by transplantation to a different medium and to have found 

 natural races exhibiting characteristics similar to those which 

 he induced, living in appropriate natural conditions. 

 Woltereck's conclusions have been seriously challenged by 

 Wesenberg-Lund, who supplies a totally different explanation, 

 and the matter must be left very largely in abeyance, with the 

 qualification that as far as Woltereck's experiments are con- 

 cerned they bear a striking resemblance in the results to those 

 of Agar. 



xvii. Sumner (1932, summary). Geographical races of 

 Peromyscus. 



Sumner conducted for many years an extensive series of 

 observations and experiments on the species and races of 

 Peromyscus (deer-mice of N. America). He has summarised 

 the work in a survey which involves the modification of views 

 previously published. As he states (1932, pp. 2-3), he started 

 the investigation ' with a distinct bias in favour of the cumula- 

 tive effect of climatic influence.' This bias was due to the 

 results of certain experiments on white mice. The animals 



