130 



A COMPARISON OF SOME PECTENS FROM THE EAST 



indices of variability, a; and the coefficient of variation, c. The left valves, since 

 they have the same symmetry as the right, were not studied. 



RIGHT VALVE. 



Expressed fully these quantitative results show that, using the coefficient of 

 variation (c) as a basis of comparison, the San Diego Pectens have an anterior partial 

 length 1.7 times as variable as those of Tampa. Also, that the posterior partial 

 length of the San Diego shells is twice as variable as the corresponding length 

 of the Tampa lot. Finally, for shells of a given anterior partial length the posterior 

 partial length is 1.5 times more variable in the case of San Diego shells than in the 

 case of Tampa shells. 



To summarize: In all proportions measured, the San Diego Pectens show them- 

 selves from 50% to 100% more variable than those of Tampa. 



VIII. CONCLUSIONS. 



Up to this stage we have not left the solid ground of ascertained fact. We must 

 now traverse the alluring but unsteady ground of logical analysis to arrive at an 

 explanation of the greater variability of the San Diego shells. 



We have dealt in the present paper only with slight, individual or, as I prefer 

 to call it, "trivial" variation, using the word in its older sense of "common" rather 

 than its newer sense of "trifling." 



1. Trivial variation is, in general, due to one or several of the following causes: 



a. The very complexity of the developmental process makes it impossible that two 

 exactly similar forms should be produced. The effect of this cause may be meas- 

 ured by comparing "identical" twins, or the offspring of the isolated blastomeres 

 of a sea-urchin or other animal. Perhaps also by comparing the different embryos 

 from one batch of eggs developed under identical conditions. We find, very fre- 

 quently, that the variability of these individuals, although sensible, is unusually small. 



