210 On some Variations of Cardyum edule. [June 6, 



become a well-marked variety before being subjected to the new 

 conditions ; while those which are found in Abu Kir must clearly be 

 the immediate descendants of animals of the type found in the 

 Mediterranean. 



Though the subject cannot be adequately discussed in an abstract, 

 it may be suggested that in so far as any variation (as, for example, 

 that of texture) occurs universally among the shells of a given 

 sample, it may be legitimately supposed that they are correlated to 

 the conditions under which they lived. 



Instances in which it is possible to actually trace the history of 

 variation under natural conditions are so rare that these observations 

 of phenomena otherwise unimportant have an increased value. The 

 opportunity given by the terraces of Shumish Kul for the comparison 

 of several distinct stages in the origin of a natural variation appears 

 to be almost unique. 



Talle' showing the Average Ratio of Length to Breadth in Shells from 

 the various Localities. 



In each case the average was taken in thirty individuals. It is 

 impossible in an abstract to give the particulars of the measurements ; 

 these appear in the fuller account. This table gives a summary of the 

 results. The extremes of length of the shells measured are given in 

 millimetres and the average breadths are given in terms of the length, 

 which is taken as 1. 



Locality. 



Level. 



Smaller 



samples. 



Larger samples. 



Extremes 

 of length 

 in mm. 



Average 

 breadth. 



Extremes 

 of length 

 in mm. 



Average 

 breadth. 



Shore of Aral Sea 





22—18 -5 



0-761 







Shuroish Kul . . . 



1st terrace 



21—17 



0-799 









2nd terrace 



21—17 



0-782 



26—19 



0-770 



Ditto 



3rd terrace 



22—18 



0-751 







Ditto 



4th terrace 



21—16 



0-735 



26—18 



0-730 





5th terrace 



21—16 



0-743 



27—21 



0-731 





7th terrace 



21-16 



-725 







Jaksi Klich .... 



Upper deposit 



22—17 



0-740 







Ditto 



Lower deposit 



25-5—19 



0-682 ' 



30—25 -5 



0-660 



Jaman Klich. . . . 



Lower deposit 



24—16 



0-726 







Sb ore of Mareotis 









27—20 



0-680 



Hamleh Lake 













No. 2 



(Fresh water) 



21—17 



0-665 



29—16 -5 



0-657 



Shore of Abu Kir 





24—19 -5 



0-738 







