1916.] natural sciences of philadelphia. 153 



Natural Selection. 



Natural selection, we can assume, is most severe on Little Duck 

 Island. Tliis little island, lying four miles out at. sea, is exposed to 

 the surf on every side. The herring gull, one of the chief enemies 

 of adult Thais, has a rookery on the island, and every summer thou- 

 sands of birds roost there at night and have their nests. Everywhere 

 regurgitated Thais shells cover the rocks, which tell a story of a 

 struggle for existence. 



In collecting on this island for two hours but four hundred and 

 twenty-nine living snails were found. These were discovered at the 

 lower edge of the Mytilus Zone. All had been picked off the rocks 

 above by the gulls. This collection is chiefly notable in containing 

 a very large proportion of dark individuals, only 1.1 per cent, white 

 were found. Nineteen per cent, were imbricated. 



It can be said with justification that where the struggle for exist- 

 ence is greatest on the exposed islands and the muddy harbors 

 selection tends to preserve the dark-colored and the lamellated 

 forms. 



Conclusions. 



A study of the color varieties leads one to the conclusion that they 

 are hereditary and that we are dealing with things that are probably 

 unit characters. Even the development of lamella?, as in imbricata, 

 are not modifications caused by induction that is caused by the direct 

 action of the environment. 



As there are great differences between the ratio of the color varieties 

 present in different localities, we assume that natural selection is 

 acting. 



In the bay environment conditions are at an optimum. No mud 

 film covers the Mytilus, and Balanus forms crusts over the rocks an 

 inch to an inch and a half thick. Here Thais is most abundant. 

 Five hundred can be gathered in ten minutes or even less. A handful 

 can be gathered at one scoop. 



In the surf environment the barnacles do not form such thick 

 crusts. On the harbor shore ice destroys the barnacles in the winter 

 and a mud film covers the mussels. So that the barnacles are usually 

 small and far apart, while the mussels are separated by mud from 

 the rocks. Sand and mud are barriers to Thais. Although the 

 surf environment and the harbor environment are the antithesis of 

 one another, yet they have many effects on Thais that are similar. 

 These effects are as follows: 



