224 



CHAPTER II 



exists, provided that the genetic constitution of the organism is sufficiently in 

 accordance with the new environment. Eurytopic ("plastic", non-demanding) 

 species, like the ecological ubiquists and the geographical cosmopolitans, are in 

 little need of genetic variability, caused either by re-combinations or by mutations 

 of genes. As has been shown by experiments, the pronouncedly diffuse thermal 

 preferendum of a eurytopic species (the Carabid beetle Pterostichus nigrita F.; 

 Lindroth, 1949, p. 50-53) may be explained by the relative insensibility of each 

 individual, and not by the presence of a genetically heterogenous population. 



And how about the parthenogenetic species, such as most Otiorrhynchus weevils 

 and some related genera, now making their triumphal progress throughout all 

 temperate parts of the world? Their prevalence over bisexual forms and species 

 among immigrants is due to the fact that the introduction of one single individual, 

 regardless of stage, is sufficient for a permanent colonization. Their genome is 

 rigid, shut out from new combinations, and there is nothing indicating that their 

 mutation frequency is higher than normal. The genetic constitution of these 

 successful parthenogenetic forms is apparently suitable and has no need of im- 

 provement. 



Extinction is not first and foremost a question of population density but of 

 genetic degeneration, accentuated by unfavourable changes of environment {vide 

 Mayr, 1944, p. 224). 



