ON VARIATIONS. 



339 



breadth than the 1895 ones, and these than the 1893 

 ones. There seems no reason to believe that the action 

 of the mud upon the beach is different from that in an 

 experimental aquarium, and hence, in Professor Wei- 

 don's opinion, there is " no escape from the conclusion 

 that we have here a case of Natural Selection acting 

 with great rapidity because of the rapidity with which 

 the conditions of life are changing." The selective de- 

 struction seems to depend on the nitration of the water 

 into the gill chambers of the crabs. To quote Pro- 

 fessor Weldon, " The gills of a crab which has died 

 during an experiment with china clay are covered with 

 fine white mud, which is not found in the gills of the 



s D 



30 



20 



10 



-50 _40 -30 -20 -10 +10 -(-20 +30 +40 



FIG. 26. Distribution of the frontal breadths of 248 male crabs, 



and of the 94 survivors, 



survivors. In at least 90 per cent, of the cases this 

 difference is very striking." Professor Weldon thinks 

 it can be shown that a narrow frontal breadth renders 

 one part of the process of filtration of water more effi- 

 cient than it is in crabs of greater frontal breadth. 



Such, then, is Professor Weldon's demonstration of 

 a particular instance of Natural Selection. In order 

 to strengthen the proof of its existence, further meas- 

 urements of crabs collected at the same spot a few years 



