Page:Popular Science Monthly Volume 78.djvu/538

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of fine china clay, while the growing cities along the shore and the shipping 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 breakwater.

PSM V78 D538 Change in the frontal breadth of carcinus.png

Fig. 3. Change in Frontal Breadth of Carcinus. The slope of the lines shows the change in mean relative frontal breadth for crabs of different 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.