DAVENPORT. — EVOLUTION OF PECTEN. 143 



snails in different parts of the same valley. The difference between any two 

 adjacent valleys is, however, very marked. 



" All the snails of Tipasrui valley are dextral, while all of the same species 

 in Pine valley are sinistral. In the two intermediate valleys of Hamuta and 

 Fautaua some individuals are dextral and some are sinistral." 



Now it is clearly absurd to ascribe the dextrality of all the snails of 

 one valley to one condition of life and the sinistrality of all the snails 

 of another valley to a different condition of life. Sinistrality and dex- 

 trality arise suddenly in various groups and are strongly inherited quali- 

 ties having no close relation to environmental conditions. 



The opposite extreme of geographic races is induced, or at least 

 influenced, by environmental conditions. Such is the case with some 

 species of North American birds which are dark in the moist northwest 

 of the United States and ash-colored in the southwest. Years ago 

 Allen (1871) pointed out that northern mammals spreading southward 

 tend to diminish in size with latitude, or as the temperature increases. 



The foregoing types of geographic races are extremes. There will be 

 in the different localities all grades of diversity of environmental effect 

 from zero up. In the case of the Pecten opercitlaris studied by me there 

 are diverse environmental conditions of temperature, depth, and density 

 of the water in the three localities, and these may all have contributed to 

 making the shells from these localities dissimilar. It will be worth while 

 to compare the results here gained with those derived from other quanti- 

 tative studies on geographic races. 



One of the first to study race differentiation quantitatively has been 

 Heiucke. His great work (1898) is, unfortunately, little known to the 

 mass of naturalists.* His measurements of thousands of herrings of the 

 North Sea and elsewhere led him to recognize that races of herrings 

 exist of which two main divisions are physiologically isolated by repro- 

 ducing in the spring and in the autumn respectively, while subdivisions 

 of these breed in particular and widely separated localities. Moreover 

 the young develop in different conditions of water density ; some in the 

 open sea and others in brackish-water harbors. Heincke is convinced 

 that the cause of the origin of the diverse herring races is a direct action 

 of dissimilar conditions of life. Similarly, Kyle (1900) has found by 

 quantitative methods that the plaice (Pleuronectes platessa, L.) from the 

 Baltic and the North Sea are distinct races, and he has been able, more- 



* For reviews of this rather inaccessible work see Duncker (1899) and Kedeke 

 (1902). 



