295 



actual standard deviation. When the relatively small size and weak 

 growth of crenate seedlings are also taken into account, the relatively 

 small average size of crenate embryos may be considered to be 

 demonstrated beyond reasonable doubt. 



With "extreme" and "ordinary" slender parents the odds de- 

 cidedly favor the hypothesis of genetic differentiation of parents, in 

 spite of the small numbers involved. We must remember that definite 

 statistical differentiation of lots of progeny grown under uniform con- 

 ditions does not necessarily demonstrate genetic differences (differences 

 in output of gametes) between the parents; in this case, however, the 

 difference in the appearance of the parents and in the single-double 

 ratio among the progeny also suggest genetic differentiation. 



GENERAL DISCUSSION 25 



It might be argued with some plausibility that the available 

 evidence hardly justifies conventional factorial analysis, or at least that 

 the data indicate strongly the presence of marked factorial incon- 

 stancy. The aberrant types occur in very small proportions among 

 the progeny of selfed Snowflake parents, in much larger proportions 

 from "mutant-type" parents, and in intermediate proportions from 

 crosses with Snowflake. It might be supposed that the Snowflake type 

 has a slight tendency to mutate to the other types, and that these have 

 a much more marked tendency to mutate back to Snowflake. Various 

 considerations, however, especially the occurrence of apparently 

 regular linkage phenomena, seem to favor the general form of 

 hypothesis which has been presented. 



As we have seen, it is well known from the behavior of various 

 factors that the typical Mendelian mechanism is present in Matthiola. 

 It cannot be argued here, as sometimes with Oenothera, that the 

 genetic behavior of the genus or species is fundamentally non- 

 Mendelian. Since the Mendelian mechanism is demonstrably present, 

 and Muller's (1918) work on beaded wings in Drosopkila seems to 

 establish the adequacy of this 'mechanism in a closely parallel case, 

 surely conventional factorial analysis should be carried as far as pos- 

 sible; in fact (Muller, 1918, p. 423), a Mendelian explanation should 

 not be abandoned for anything short of positively contradictory 

 evidence. 



2. r > Muller's (1918) complete report on the beaded-wing case in Drosophila 

 appeared several months after the present paper had gone to the publisher. 

 Certain conclusions given below, very similar to Muller's but not credited to 

 him, were therefore reached independently. 



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