898 



REPORT — 1898. 



Table IV. — The Mean Frontal Breadth Ratio of Male Carcinus mcenaa/rw/i a 

 particular patch of beach in Plymmdh, in the years 1893, 1895, and 1898. 



These results all relate to male crabs. The change in female crats during this 

 time has been less than the change in male crabs, but it is, so far as my 

 measurements at present permit me to speak, going on in the same direction as the 

 change in male crabs. 



I think there can be no doubt, therefore, that the frontal breadth of these crabs 

 is diminishing year by year at a rate -which is very rapid, compared with the rate 

 at which animal evolution is commonly supposed to progress. 



1 will ask your patience for a little while longer, that I may tell you why I feel 

 confident that this change is due to a selective destruction, caused by certain 

 rapidly changing conditions of Plymouth Sound. 



If you look at the chart, you wiU see that Plymouth Sound is largely blocked 

 up, and its communication with the sea is narrowed by a huge artificial break- 

 water, about a mile long, so that the tidal currents enter it and leave it only by 

 two openings. This huge modem barrier has largely changed the physical 

 conditions of the Sound. 



On either side of Plymouth itself a considerable estuary opens into the Sound, 

 and each of these estuaries brings down water from the high granite moorlands, 

 where there are rich deposits of china clay. Those of you who know Dartmoor 

 will remember that in rainy weather a great deal of chma clay is washed into the 

 brooks and rivers, so that the water frequently looks white and opaque, like milk. 

 Much of this finely divided china clay is carried down to the sea ; and one efiect of 

 the breakwater has been to increase the quantity of this fine silt which settles in 

 the Sound itself, instead of being swept out by the scour of the tide and the waves 

 of severe storms. 



So that the quantity of fine mud on the shores and on the bottom of the Sound 

 is greater than it used to be, and is constantly increasing. 



