338 ACTION OF NATURAL SELECTION 



destruction, caused by certain rapidly changing condi- 

 tions in Plymouth Sound. Owing to the building of a 

 huge breakwater, the scour of the tide has been dimin- 

 ished, and the large quantities of china clay carried 

 down by the rivers from Dartmoor into the Sound 

 therefore settle in increasing quantities in the Sound 

 itself. Also the quantity of sewage and refuse finding 

 its way into the Sound is steadily increasing, owing to 

 the increase in the size of the contiguous towns and 

 dockyards. " It is well known," says Professor Wei- 

 don, " that these changes in the physical condition of 

 the Sound have been accompanied by the disappearance 

 of animals which used to live in it 2 but which are now 

 found only outside the area affected by the break- 

 water." In order to test his supposition of selective 

 destruction, Professor Weldon placed a number of 

 crabs in a large vessel of sea water, in which a consider- 

 able quantity of very fine china clay was suspended. 

 The clay was prevented from settling by a slowly mov- 

 ing automatic agitator. After a time, the dead crabs 

 were separated from the living, and both were meas- 

 ured. In the figure given below is shown the result 

 obtained. 



Here the upper curve shows the distribution of fron- 

 tal breadths of the 248 male crabs experimented on, and 

 the dotted curve the distribution of frontal breadths 

 of the 94 survivors. The line represents the mean 

 frontal breadth of all the crabs, the dotted line S the 

 mean of the survivors, and the dotted line D the mean 

 of the dead crabs. The crabs which survived thus had 

 a distinctly smaller frontal breadth than those which 

 were killed, just as the 1898 crabs had a smaller 



