378 



The Measurable Characteristics of Plants, $c. [Feb. 28, 



margin, measured from the tip of the first to the tip of the last visible 

 tooth, would really have been measured from a to 6. Such a measure 

 would not be homologous with the rest, and ought not to be included 

 in the scheme. But, since reduction in the number of teeth may take 

 place in other ways, and since it is impossible in a given case to 

 distinguish the manner in which it has occurred, the measurements 

 were necessarily recorded. 



Another frequent cause of disturbance is the breakage of the last 

 tooth, followed by its regeneration. All cases of obviously recent- 

 injury were of course excluded ; and for this reason the total number 

 of individuals discussed is reduced from 8069 to 8020. But the 

 selection of material was felt to be so dangerous a proceeding that 

 all cases in which there was any doubt as to the occurrence of an 

 mjury were included. The wrongful inclusion of a dozen such cases 

 would account for the excess of positive abnormalities : for it is 

 evident that a breakage of the tip of the last tooth would increase 

 the distance AB in fig. 1. 



While, therefore, the observations admit of the interpretation that 

 about once in 500 cases a " sport " of magnitude greater than that 

 provided for by the theory of chance does regularly occur, the con- 

 siderations which have been submitted make this interpretation at 

 least doubtful. 



The value of the probable error, as an indication of percentage 

 abnormality, is diminished by the existence of the large deviations 

 discussed ; but the values obtained are of considerable interest : they 

 are as follows : 



Table VI. Mean Value of Quartile Deviation (Q) of Right Dentary 

 Margin for various Lengths of Carapace (C). 



It will be seen from this table that the error of distribution at the 

 ages measured is always less than in adult life, except among crabs, 

 whose carapace length is between 9 and 10 mm. Of the fourteen 

 superfluous deviations of great magnitude, three occur in this group, 

 and the result is a quite untrustworthy determination. 



