900 . REPORT— 1898. 



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, precisely as 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 diminish- 

 ing 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 increasing in amount, and which can be shown 

 experimentally to 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 experimental 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 destruction 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 have as good a chance of life as the rest ; and in consequence 

 the protected crabs should contain a larger percentage of broad individuals than wild 

 crabs of the same age ; and the mean frontal breadth of such a protected popula- 

 tion ought to be greater, after a little time, than the mean frontal breadth of wild 

 crahs, 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 numbered glass 

 bottles, each bottle being provided with a constant supply of clean sea-water by 

 means of a system of glass syphons. Into each of these bottles I placed a crab 

 from the beach. After a considerable number of deaths had occurred, a series of 

 crabs was finally established, each crab living in a numbered bottle, until it had 

 cast its shell. The process of moulting involves no distortion of the carapace, 

 which could affect the measurements concerned, and therefore each cast shell was 

 carefully measured. The measurements of these shells were carefully compared 

 with measurements of wild crabs of the same size, and the mean frontal breadth 

 of these shells was a little less than the mean breadth in wild crabs of corresponding 

 length.' 



After each crab had moulted, it was left in its bottle until it had grown and 



' This was probably due to the death-rate during acclimatisation being selective. 

 It was very difficult to keep the apparatus clean ; and the deaths which occurred 



