No. 607] 



RATS AND EVOLUTION 



391 



one explain that those groups of plants, which are so 

 constituted that they become automatically pure in a short 

 number of generation— the autogamous plants— con- 

 sist in the main of pure and invariable species, which can 

 not be changed by any amount of selection.^ Selection 

 within a group of plants which descends from one indi- 

 vidual, homozygous for all its genes by a continued auto- 

 fecundation, is ineffective. As we have the name "pure 

 line" for these groups of plants, there is no good reason 

 to limit the use of the term ''species" to these groups 

 exclusively. 



Liability to change by selection is synonymous with 

 genotypic variability, and this true variability is sjnony- 

 mous with impurity. Those species which do not exist 

 exclusively of individuals which are all mutually identical 

 in respect to all their genes, are variable and therefore 

 liable to change by selection. One single, genot\^ically 

 pure species as a rule can not give rise to new species. 

 There have become known a few cases^ of real spon- 

 taneous genovariation, mutation, in which every known 

 cause for change in genotype was excluded (one of us has 

 noted three such instances in the mouse) ; but as in every 

 instance we have been concerned with a dropping out of 

 one gene we can practically leave them out of account 

 here. There exist pure species, but there certainly also 

 exist variable species, species which are certainly liable 

 to change by selection. 



In evolution we are certainly concerned with two dif- 

 ferent sets of processes, on the one hand with the causes 

 of variability, and on the other hand with the processes 

 which limit variability. 



Throughout this paper we will call total potential 

 variability the quantity of genes which not all the mem- 



* A. C. Hagedoorn and A. L. Hagedoorn, ' ' Studies on Variation and Se- 

 lection," Zeitschr. fur Induktive Abstammungs- und Vererhungslehre, 1914. 

 A. C. Hagedoorn and A. L. Hagedoorn, "CJan Selection imj^rove the Quality 

 of a Pure Strain of Plants?" Journal of the Board of Agriculture. 1914. 



5 A. L. Hagedoorn, "The Genetic Factors in the DevelopnitMit of the 

 House-mouse," Zeitschrift fur Induktive Abstammu)!;/.^- uini V < v, rhungs- 

 lehre, 1911. 



