14 THE MEASUREMENT OF VARIATION. 



case but one to conform to that required by the Law of 

 Error. The single exception to the general rule will 

 be referred to in the next chapter. Again, H. Thomp- 

 son * made twenty-two different measurements on 1000 

 adult prawns, and found the variations in every case but 

 one to correspond more or less accurately with the law. 

 E. Warren f made seven different measurements on 

 2300 male crabs (Portunus depurator), obtained from 

 Plymouth, and found that the variations very nearly 

 corresponded to the law. Duncker % made eight series 

 of determinations on the number of spines and rays in 

 the fins of 1900 specimens of the fish Acerina cernua, 

 and found that with one slight exception the variations 

 obeyed the general law. Of the twelve series of meas- 

 urements made on 1120 specimens of the flounder 

 (Pleuronectes flesus), however, only six were quite sym- 

 metrical and in accordance with the law. Finally the 

 author || made 9850 measurements on the plutei or 

 larvae of a sea-urchin, Strongylocentrotus lividus, and 

 found that the variations in size corresponded very 

 closely indeed with the law. The lengths of the arms 

 of these plutei were calculated as percentages on the 

 length of body, and were found in the case of the oral 

 arm lengths to correspond closely with theory, but in 

 the case of the anal arm lengths, there was some slight 

 divergence. 



With regard to the variation of plants, our accurate 



* Proc. Roy. Soc. , Iv. p. 234, 1894. 



fProc. Roy. Soc., Ix. p. 221. 



\ Biologischen Centralblatt, xvii. p. 785, 1897. 



gZool. Anzeig., xxiii. p. 141. 



|| Phil. Trans. 1895, B. p. 613. 



