September 22, 1898] 



NATURE 



505 



destruction, caused by certain rapidly changing conditions of 

 Plymouth Sound. 



If you look at the chart, you will see that Plymouth Sound is 

 largely blocked up, and its communication with the sea is 

 narrowed by a huge artificial breakwater, about a mile long, so 

 that the tidal currents enter it and leave it only by two openings. 

 This huge modern 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 re- 

 member 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 

 bottom 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 in 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 animals 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 col- 

 lected and placed 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 moving automatic 

 agitator ; and the crabs were kept in under these conditions for 

 various periods of time. At the end of each e.'cperiment the 

 dead were separated from the living, and both were measured. 



In every case in which this experiment was performed with 

 china clay as fine as that brought down by the rivers, or nearly 

 so, the crabs which died were on the whole distinctly broader 

 than the crabs which lived through the experiment, so that a 

 crab's chance of survival could be measured by its frontal 

 breadth. 



When the eicperiment was performed with coarser clay than 

 this, the death-rate was smaller, and was not selective. 



I will rapidly show you the results of one or two experiments. 

 The diagram (Fig. 5) shows the distribution of frontal breadths, 

 about the average proper to their length, in 248 male crabs 



divided china clay does kill crabs in such a way that those in 

 which the frontal breadth is greatest die first, those in which it 

 is less live longer. The destruction is selective, and tends to 

 lower the mean frontal breadth of the crabs subjected to its 

 action. It seemed to me that the finer the particles used in the 

 experiments, that is to say, the more nearly they approached the 

 fineness of the actual silt on the beach, the more selective their 

 action was. 



I therefore went down to the beach, where the crabs live, and 

 looked at the silt there. This beach is made of moderately 

 small pieces of mountain limestone, which are angular and little 

 worn by water. The pieces of limestone are covered at low tide 

 with a thin layer of very fine mud, which is much finer than the 

 china clay I had used in my experiments, and remains suspended 

 in still water for some time. Under these stones the crabs live, 

 and the least disturbance of these stones raises a cloud of very 

 fine mud in the pools of water under them. By washing the 

 stones of the beach in a bucket of sea water, I collected a 

 quantity of this very fine mud, and used it in a fresh series of 

 experiments, preciselyas I had before used china clay, and I 

 obtained the same result. The mean frontal breadth of the 

 survivors was always smaller than the mean frontal breadth of 

 the dead. 



I think, therefore, that Mr. Thompson's work, and my own, 

 have demonstrated two facts about these crabs ; the first is that 

 their mean frontal breadth is diminishing year by year at a 

 measurable rate, which is more rapid in males than in females ; 

 the second is that this diminution in the frontal breadth occurs 

 in the presence of a material, namely, fine mud, which is in- 

 creasing in amount, and which can be shown experimentally to 



Fig. 5. — Diagram showing the eft"ect of china clay upon 248 male crabs. The upp;r curve shows the 

 distiibution of frontal breadths in all these crabs; the dotted curve the distribution of frontal breadths 

 in the survivors. The dotted line s shows the mean of the survivors ; the line d the mean of the dead. 



destroy broad-fronted crabs at a greater rate than crabs with 

 narrower frontal margins. 



I see no shadow of reason for refusing to believe that the 

 action of mud upon the beach is the same as that in an experi- 

 mental aquarium ; and if we believe this, I see 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. 



Now, if we suppose that mud on the beach has the same effect 

 upon crabs as mud in an aquarium has, we must suppose that 

 every time this mud is stirred up by the water, a selective de- 

 struction of crabs occurs, the broad-fronted crabs being killed in 

 greater proportion than the narrow-fronted crabs. 



Therefore, if we could take a number of young crabs, and 



protect them through a certain period of their growth from the 



action of this selective mud, the broad-fronted crabs ought to 



treated in one ex"perimenL Of these crabs, 154 died during the j have as good a chance of life as the rest ; and in consequence 



experiment, and 94 survived. The distribution of frontal 

 breadths in the survivors is shown by the lower curve in the 

 diagram, and you see that the mean of the survivors is clearly 

 below the mean of the original series, the mean of the dead 

 being above the original mean. 



Two other cases, which are only examples of a series in my 

 po.sses«ion, show precisely the same thing. ^ 



These experiments seemed to me to show that very finely 



1 It is impossible in this place to give a full account of the experiments 

 referred to, and a multiplication of mere small scale diagrams seems useless, 

 so that only one of those exhibited when the address was delivered is here 

 reproduced. 



the protected crabs should contain a larj^er percentage of broad 

 individuals than wild crabs of the same age ; and the mean 

 frontal breadth of such a protected population ought to be 

 greater, after a little time, than the mean frontal breadth of 

 wild crabs, in which the broad individuals are being constantly 

 destroyed. 



It is difficult to perform this experiment, because one cannot 

 know the age of a crab caught on the shore. But so far as one can 

 judge the age of a crab by its length, I can show you that the 

 thing which ought to happen, on the hypothesis that such 

 selective destruction is going on, does actually happen. 



I established an apparatus consisting of some hundreds of 



NO. 1508, VOL. 58] 



