6 Mutations and Evolution. 



CHAPTER II. 

 FOUNDATIONS OF THE MUTATION CONCEPT. 



In later chapters we shall deal with the occurrence of 

 mutations in a wide range of wild plants and animals. 

 The writer's book on mutation 1 , dealt particularly with the 

 mutations of (Enothera and the conclusions to he drawn from 

 them. A cell theory of mutations was formulated, hased on the 

 cytological and breeding work with (Enothera and incident!) 1 a 

 wide range of other forms ; and concepts were developed which 

 proved applicable to many other plants and animals. This has 

 involved certain departures from the theory of mutations as 

 originally propounded by deVnes. Instead of the purely 

 hypothetical and conceptual pangens, whose alterations were the 

 putative cause of mutations, it has been possible to link up many 

 of the changes in (Enothera with visible structural changes in 

 the chromatin of the cell. 



Let us consider for a moment the simplest case of this kind. 

 It is already well-known, but requires restatement because some 

 writers have endeavoured to deny the significance of the facts. 

 The lata mutation is now known to occur in (E. lamarckiann, 

 (E. biennis and (E. suaveolens. The peculiarities are the same in 

 every case obtuse-tipped, deeply crinkled leaves, a somewhat 

 weak or irregular habit of growth, stout rounded buds, almost 

 completely sterile anthers, and 15 chromosomes in the nuclei. This 

 constellation of characters is super-imposed on, or rather substituted 

 for, the characters of the species in each case, and the same 

 mutation has also been observed from some of the mutants of 

 (E. lamarckiana and also in various hybrids. Thus (E. biennis 

 mut. lata has small flowers while (E. lamarckiana mut. lata has 

 large flowers, but the lata forms otherwise agree. (E. lata 

 rubricalyx, two of which occurred in the F 2 ofrubricalyxxgrandiflora 

 had lata peculiarities combined with the red pigmentation of 

 rubricalyx? It is also significant that lata mutants sometimes 

 occur in pairs. 



Only one hypothesis based on observation has been suggested 

 to account for the origin of lata with its 15 chromosomes. In 

 1908 occasional irregular reduction divisions in the pollen 

 mother cells were observed in CE. mut. rubrinervis whereby an 8-6 

 separation of chromosomes took place in the heterotypic mitosis. 

 This would lead to the formation of two pollen grains with 8 



1 Gates, 1915. ' Gates, 1914; Gates and Thomas, 1914. 



