DARWINISM DEFENDED. 159, 



crabs 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 com- 

 monly supposed to progress. 



"I will ask your patience for a little while longer, that I may 

 tell you why 1 feel confident that this change is due to a selective 

 destruction, caused by certain rapidly changing conditions of 

 Plymouth 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 china 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 effect 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 bot- 

 tom of the Sound is greater than it used to be, and is constantly 

 increasing. 



"But this is not all. During the forty or fifty years which have- 

 gone by since the breakwater was completed, the towns on the 

 shores have largely increased their population ; the great dockyard 

 at Devonport has increased in size and activity; and the ships 

 which visit the Sound are larger and more numerous than they 

 were. Now the sewage and other refuse from these great and 

 growing towns and dockyards, and from all these ships, is thrown 

 into the Sound; so that while it is more difficult than it used to be 

 for fine silt to be washed out of the Sound, the quantity thrown into 

 it is much greater than it was, and is becoming greater every day. 



"It is well known that these changes in the physical conditions 

 of the Sound have been accompanied by the disappearance of ani- 

 mals which used to live in it, but which are now found only outside 

 the area affected by the breakwater. 



"These considerations induced me to try the experiment of keeping 

 crabs in water containing fine mud in suspension, in order to see 

 whether a selective destruction occurred under these circumstances 

 or not. For this purpose, crabs were collected and placed in a large 

 vessel of sea-water, in which a considerable quantity of very fine.- 



