Xo.597] 



515 



its morphology or by its physiological behavior. In 1912 

 de Vries and the speaker visited the locality from which 

 Davis's, strains came, and found such a confusion of dif- 

 ferent types growing together that it was impossible to 

 doubt that the entire (Enothera population was hybridized 

 to a greater or less degree. Some of the forms belonged 

 to the series of (E. grandiflora, being large-flowered and 

 open-pollinating, whereas others were small-flowered, self- 

 pollinating types, showing obviously the effects of hy- 

 bridization with (E. Tracyi, another southern species. It 

 is curious that the particular (Enothera to be put for- 

 ward as probably genetically pure, judged by Davis's 

 etiological criterion, or by Jeffrey's pollen test, should 

 come from a locality where hybridization is so prevalent 

 that one would hardly expect to find among the open- 

 pollinating forms a single unhybridized plant. It will be 

 remembered that Jeffrey has attacked the mutation theory 

 from the point of view that pollen abortion necessarily 

 indicates hybridity. By this criterion the small-flowered 

 practically cleistogamous species which self-pollinate 

 generation after generation must be adjudged highly im- 

 pure, although the evidence is all to the contrary. There 

 is every indication that pollen abortion is a frequent con- 

 comitant of mutation, as well as of hybridization. It 

 seems not unlikely, therefore, that the unpaired condition 

 of the meiotic chromosomes may have a causal relation- 

 ship with pollen abortion as well as with the production 

 of those types of mutations which have a chromosome 

 number different from that of the parent species. 



It is ordinarily supposed that a mutation is determined 

 when the reduction division takes place. This may be the 

 case with the mutations in which there are irregular 

 chromosome numbers. Even in such cases it may be that 

 the germ plasm has undergone at some other point in the 

 life cycle a premutative modification of such a nature as 

 really to predetermine the kinds of mutated gametes 

 which will subsequently appear. A physiological premu- 

 tation might, for instance, bring about the condition 

 which results directly in loose chromosome pairing, and 



