Evolution without Mutation. 



139 



. . . which had a wide range on a continent, the variation 

 exhibited by individuals from different locaHties is generally so 

 considerable that it is hardly possible to predict its amount while 

 almost every intermediate form may be found if the series of 

 specimens be large enough." 



To get additional data relating to geographical variation I have 

 made quantitative studies of races of Pecten inhabiting different 

 geographical regions. My first example will illustrate the fact of 

 geographical variation where the extremes are not usually called 

 species. Pecten opercularis occurs on the coast of Europe from 

 the Lofoten Islands to the Canary Islands and throughout the 

 Mediterranean Sea. I have studied (1903) specimens from three 

 localities on the coast of Great Britain: at Eddystone Light, the 

 Irish Sea, and the Firth of Forth, at north latitudes 50° 15', 

 54° 18' and 56° 05^ respectively. I find that the individuals from 

 the ends of the series are the most unlike and that those from the 

 intermediate latitudes are intermediate in most of their dimen- 

 sions, as the following table shows: 



Eddystone. 



Irish Sea. Firth of Forth. 



70 mm. 

 1.067 



Maximum dorso-ventral diameter 

 Ratio ant. -post, to dorso-ventral 



diameter when latter = 67 mm . . 

 Ratio hinge length to antero-pos- 



terior diameter 



Half globosity at length of 53 mm, 

 Relative length of ears to hinge. . , 



Average number of rays , 



Standard deviation of rays 1 . 000 ± . 020 



Coefficient of variability 



77 mm. 

 1. 061 



•039 



This series shows that there is a geographical variation and that 

 the transition from one extreme to the other may be gradual. 



The second example has to do with the Pectens of the East 

 coast of the United States. On the shores of Long Island is a 

 species of scallop known as Pecten irradians. On our Gulf of 

 Mexico coast occurs a second "species"— P. gibbus. For the 



