DAVENPORT. — EVOLUTION OF PECTEN. 145 



snails, Littorina littorea, from different localities of Great Britain and 

 Nortli America, have a proportion of breadth to height of shell varying 

 from 88 to 92.5 per cent, and as Duncker (1898) has pointed out, the 

 shells from Great Britain are in all cases much less variable (o- = 2.3) 

 than those from North America (o- = 2.4 to 3.0). Recently Miss Dimon 

 (1902) has shown in Biometrika that the absolute dimensions and pro- 

 portions of the mud snail, Nassa, vary in different localities of the same 

 harbor, and in this case the shells are subjected to diverse environmental 

 conditions. 



Bateson (1889), studying cockles from old beaches of southwest 

 Asiatic seas that are gradually becoming salter through evaporation, has 

 demonstrated most clearly a change in proportions of the shell with 

 increasing salinity. 



In some studies on Pecten irradians of our coast from Long Island, New 

 York, to Pensacola, Florida, I was struck by the gradual change of the 

 shells from place to place ; a change of such a nature that one might say 

 that the difference in the place modes was a function of the spacial inter- 

 val between the places in question. This first study on the pectens of 

 Europe yields a similar result. Three lots of one species collected at 

 three points on the coast of Great Britain are measurably unlike in size, 

 proportions, and average number of rays. And when the lots are 

 arranged in the order a, b, c, in which a and c are the geographical 

 extremes, they are found to be the biological extremes also. 



Lastly, the evidence from the shells we have examined bears upon De 

 Vries's law of mutation. Where the environmental conditions of the 

 isolated form units are similar the differences met with are easily 

 accounted for on the assumption of mutations which are preserved. 

 "Where, on the other hand, the environmental conditions are dissimilar it 

 is obvious that they must produce a change either through their " direct 

 and definite " action * or possibly by selection. To deny that environment 

 may act directly to produce profound, eventually specific changes is to 

 deny the evidence of some of the best experimental work in evolution, 

 and this experimental work has also proved the inheritance of these 

 environmentally induced changes. The mutation theory errs, then, in 

 stating only a half truth. Through mutation, and also through the direct 

 action of environment, specific changes may be produced. 



Hull Zoological Laboratokt, 

 University of Chicago, April 25, 1903. 



* In the sense of Darwin in his Origin of Species, chapters I and V. 

 vol. xxxix. — 10 



