TRANSACTIONS OF SECTION D. 



901 



had hardened a new shell. It was then killed and measured, and the measure- 

 ments obtained were compared with measurements of wild crabs of corresponding 

 size. This time the captive crabs were unmistakably broader than wild crabs of 

 their own size, and there were a few of the protected crabs which were very 

 remarkably broad. The distribution of abnormalities before and after moulting is 

 shown in fig. 6. 



Fig. 6. 



+ 30 +40 



+ 50 



Distribution of abnormality of frontal breadth ratios in 527 female crabs before and 

 after moulting in captivity. The continuous line shows the distribution before, 

 the dotted line after moulting. 



This is precisely the result which we ought to have obtained if the hypothesis 

 suggested by the study of mud were true. By protecting crabs through a period 

 of their growth, we ought to raise the mean frontal breadth, and to obtain a 

 greater percentage of abnormally broad crabs, and that is what we have seen to 

 occur. 



Of course, this experiment by itself is open to many objections. The estimate 

 of age by size is a dangerous proceeding, and it is difficult to exclude the possibility 

 that confinement in a bottle may directly modify a crab during the critical period 

 of moulting, and so on. All these points would have to be discussed at greater 

 length than your patience would bear, before we could accept this experiment by 

 itself as a proof that some selective agent exists on the shore which is absent from 

 the bottles. At the same time, the result of this experiment is exactly what we 

 should expect to find if such a selective agent did exist, and so it is in complete 

 harmony with the evidence already put before you. 



Of course, if the observed change in frontal breadths is really the result of selec- 

 tion, we ought to try to show the process by which this selection is eflected. 



This process seems to be largely associated with the way in which crabs filter 

 the water entering their gill-chambers. 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 survivors. In at least ninety per cent, of the cases] this differ- 

 ence is very striking ; and the same difference is found between the dead and the 

 survivors in experiments with mud. 



I think it can be shown that a narrow frontal breadth renders one part of the 

 process of filtration of water more efficient than it is in crabs of greater frontal 

 breadth. 



It would take too long to go into that matter now, and I shall not attempt to 

 do so. I will only now ask you to consider one or two conclusions which seem to 

 me to follow from what I have said. 



were in most cases due to the presence of putrescent bits of food, jchich had not 

 been removed. 



A subsequent experiment was made with the same apparatus, in which crabs were 

 kept in putrid water until a large percentage bad died ; and the mean frontal breadth 

 of the survivors was found to be distinctly less than the mean frontal breadth of 

 the dead. 



