4 Day, Variation in a Carboniferous Bracliiopod. 



and from these measurements the required ratios were 

 obtained. 



The actual limits of variation have already been 

 mentioned, and are indicated on PL i A — F, 



To indicate the distribution of variation, distribution 



graphs were constructed for the ratios -= and — (see 



b D 



Fi?. i. — Curve showing the distribution of the ratio — — ^r— 

 6 fa Breadth 



anions iooo individuals of Reticularia Hneata. 



280 





240 



- 



200 



- 



160 





120 





So 





40 



- 





"j 



I "2 



Text-figs. 1 and 2), the ratios being taken as abscissae and 

 numbers of individuals as ordinates. 



Fig. 1 is the curve showing the distribution of ' 



b 



among one thousand shells. The curve indicates that 

 there is a maximum of shells possessing a mean " z ratio 



