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THE POPULAR SCIENCE MONTHLY 



of fine china clay, while the growing cities along the shore and the ship- 

 ping have greatly increased the refuse thrown into the harbor. The 

 construction of a huge artificial breakwater has minimized the scour of 

 the tide and the waves of severe storms which formerly swept the fine 

 silt out of the sound, so that this was constantly increasing in amount 

 during the period. These physical changes affected the fauna and some 

 organisms disappeared and were only to be found outside the break- 

 water. 



Fig. 3. Change in Frontal Breadth of Carcinus. The slope of the lines shows the 

 change In mean relative frontal breadth for crabs of dlfTerent length of carapace. 



As a first series of experiments Weldon put crabs in a large vessel 

 of sea water in which a quantity of fine china clay was kept from 

 settling by a slow automatic agitator. After a period of time both dead 

 and living individuals were measured. In every case the crabs which 

 died were on the whole distinctly broader than those which lived 

 through the experiment, so that a crab's chance for survival could be 

 measured by its frontal breadth. When the experiment was performed 

 with clay coarser than that brought down by the rivers the death rate 

 was smaller, and was not selective. 



