322 Genetics of a Daphnia Hybrid during Parthenogenesis 



these animals. Since, as described above, the relative length of the 

 abdominal processes does not attain its final value till after maturity, 

 these animals were rejected. 



One other individual (in generation 3) was rejected because its 

 abdominal process was bent and deformed, so that the value of the ratio 

 (12'7.5) obtained by measuring the process along the line shown in 

 Fig. 1 could not properly be compared with those of normal specimens. 



The two remaining individuals which were omitted were one specimen 

 in generation 3 with a ratio of 23'75 (marked a in Table VI) and one in 

 generation 9 with a ratio of 16"75 (6 in the Table). These individuals — at 

 any rate the first one — obviously fall outside of the normal distribution 

 of the population and could not be properly employed in calculating the 

 statistical constants. They are probably of no genetic significance. They 

 were both very small specimens — indeed they were the smallest Xc(^'s 

 measured, with the exception of one other which was of the same size as 

 these two. As we have seen, even after sexual maturity is attained, the 



value of the ^ ratio continues to sink with increase in age, and hence size, 



of the animals, and hence there is an organic correlation between small 

 size and high ratio. 



It will be noticed that it is a general characteristic of the distri- 

 butions — not only of the hybrid clone, but of the pure species as well — 

 to be more or less markedly skew ; the range of variation generally 

 stretches out further on the high than on the low side of the mean or 

 mode. This is doubtless an expression of the fact that favourable con- 

 ditions produce large animals and hence low ratios. Now favourable 

 conditions imply a combination of environmental factors each differing 

 but little from its optimum condition, and hence variations in the environ- 

 mental complex towards the side of extreme unfavourableness are more 

 likely to occur than variations towards the other extreme of the range. 

 Hence the skewness of the distribution of the ratio values is probably 

 simply an expression of the skew distribution of the degrees of favourable- 

 ness of the environmental complex, and I think there is little doubt 

 these two specimens were merely extreme examples of environmental 

 modification. 



Another argument against the view that these two specimens are the 

 result of segregation is that the males of clone H (which these specimens 

 approach) are as regards bodily size all smaller either than any RR male, 

 or than the mean of the Xc males. The two aberrant males under dis- 

 cussion, though not reaching the high ratio of the clone H males, were 



