160 DARWINISM TO-DAY. 



china clay was suspended. The clay was prevented from settling 

 by a slowly moving automatic agitator ; and the crabs were kept 

 under these conditions for various periods of time. At the end 

 of each experiment 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 experiment 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 [omitted] shows the distribution of frontal breadths, 

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

 in one experiment. Of these crabs, 154 died during the experiment, 

 and 94 survived. The distribution of frontal breadths in the sur- 

 vivors 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 

 possession, show precisely the same thing. 



"These experiments seemed to me to show that very finely 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 do\vn 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. 



