4 o / T. CUNNINGHAM [january 



agitator. After various periods of time the dead crabs were separated 

 from the living, and the mean frontal breadth of those that died was 

 distinctly greater than that of the survivors. 



A similar result was obtained when crabs were treated in the same 

 way with fine mud obtained from the shore on which they were found. 

 Professor Weldon concludes that the action of mud on the beach is 

 the same as in the experiment, and that here we have a case of natural 

 selection acting with great rapidity because of the rapidity with which 

 the conditions of life are changing. 



Lastly, there is another experiment in which some hundreds of 

 crabs were kept, each in a separate bottle until it moulted, when of 

 course it became larger. At first some crabs died and these were 

 found to be those with broadest frontal regions. This was therefore 

 a selective destruction without the influence of mud, and was proved 

 by a separate experiment to be due to the putrefaction of the particles 

 of food with which the crabs were fed. So that organic putrefaction 

 and bacteria have the same effect on the crabs as inorganic sediment. 

 The surviving crabs moulted, and after they had hardened their new 

 shells they were killed and measured, and their dimensions compared 

 with the dimensions they had before the moult, as ascertained from 

 the cast shells. Before the moult the mean frontal breadth was below 

 the normal for the size, afterwards it was above the normal for the 

 new size, and some specimens were very remarkably broad. The 

 reason for this is stated to be that the same crabs were measured at 

 the two sizes, while in nature during the interval the broadest speci- 

 mens had been killed off. 



With regard to the experiments with china clay and mud, I 

 would point out that the smallest and youngest crabs are the broadest 

 fronted, and that the result might easily be explained on the 

 hypothesis that the smallest and youngest crabs succumbed first. 

 Professor Weldon believes that death occurs because a narrow frontal 

 breadth renders the filtration of the water entering the branchial 

 chamber more efficient. But filtration would depend on the absolute 

 size of the apparatus, and this is getting larger as the crab grows 

 larger ; therefore the larger crabs ought to be killed first, and in nature 

 all the adults ought to be killed. On the other hand, if relative 

 frontal breadth has the importance supposed, the selective destruction 

 ought to be greatest on the very young crabs, and if it kills the 

 broadest fronted of those between 10 and 15 mm. long, it ought to 

 kill all those under 10 mm. in length, since these are all broader. In 

 this case there would be no crabs over 10 mm. in length. 



In fact, Professor Weldon argues that selective destruction has in 

 five years produced a diminution of not more than 18 units in 

 the frontal breadth of crabs of the same size, while his observations 

 show that there are living on the same shore at the same time 

 multitudes of crabs of different sizes whose frontal breadths differ by 



