Evolution without Mutation. 141 



as Pecten eboreus of Conrad. I gathered shells from three 

 layers, at i foot, 4 to 6 feet and 15 feet above tide water. As 

 there were relatively few from the middle layer these shells are 

 not considered here. These layers represent periods of time, the 

 upper layer of molluscs having lived latest. 



First, let us examine the number of rays in the right and left 

 valves from the bottom and top layers and, for comparison, in 

 recent shells (1900) from Morehead, N. C, in default of living 

 Pectens from a nearer locality. Both averages (A) and indices 

 of variability (ff) are given, although the former alone are of special 

 importance in this discussion. The number of shells measured 

 in each case is given in the column headed «. 



Number of Rays. 



Lowest tier 164 



Upper tier 163 



Morehead 449 



. Transverse diameter t-. t-. ^ 



Globosity of Valve — i. e., ; when Dorso-ventral Diameter is 57-61 mm. 



Dorso-ventral diameter 



The number of varieties (n) is fairly satisfactory for deter- 

 mining the number of rays because this quality is independent 

 of the size (age) of the shell and consequently shells of all sizes 

 were taken. The proportions of shell diameters, on the contrary, 

 change greatly with age; moreover, it seemed desirable to take 

 shells of the same size from all series whether the modal size was 

 small or great. Consequently n is small in the ratio determina- 

 tions and the probable errors correspondingly high. Despite the 

 large size of the probable errors there is a significant difference in 



