MEASUREMENT OF NATURAL SELECTION 529 



By washing the stones under which the crabs live along the beach 

 Professor Weldon obtained a silt of a finer texture than the china clay 

 he had been able to use. This was employed in experiments of the same 

 kind, with identical results. 



There seems no reason to suppose that the relation of the crabs to 

 the mud on the beach is different from that in the aquarium. When- 

 ever the fine sediment is stirred up a selective elimination of crabs must 

 occur. It is this selective elimination which "Weldon regarded as fur- 

 nishing the explanation of the decrease in frontal breadth observed in 

 the measurements in 1903, 1905 and 1908. 



Not content with these experiments, Weldon tried to obtain evi- 

 dence of an entirely different kind. He arranged several hundred 

 aquaria in each of which a young crab from the beach was kept in clear 

 running sea water — and so entirely free from the influence of the mud. 

 They were allowed to moult and grow and harden new shells. When 

 measured they were found to be unmistakably broader than wild crabs 

 of the same length. This is precisely the result to be expected if a selec- 

 tive elimination of broad-fronted individuals occurs in nature. 



The source of this difference in capacity for survival seems to lie in 

 the way in which the crabs filter the water entering their gill chambers. 

 Professor Weldon found 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. The gills of the crabs which died 

 during the experiments were covered with fine white mud, and this 

 was not found in the gills of the survivors. 



The labor of these experiments — the daily care of hundreds of 

 animals, the thousands of measurements and the drudgery of calcula- 

 tion — was excessive. Most discouraging of all, perhaps, were the sterile 

 and hostile criticisms which are so often the portion of a pioneer. 



Observations on Other Invertebrates. 



Besults which may be logically attributed to the action of natural 

 selection but which by reason of the possibility of other explanations 

 are not conclusive evidence for its potency, have sometimes been se- 

 cured by biometricians concerned with other problems. For instance, 

 Warren^* adduces " the elimination of the physically unfit " as one of 

 the factors to account for the difference in variability of the termites of 

 the same nest at different seasons. Possibly this factor may also ac- 

 count in part for differences in variability from nest to nest, but of 

 course much more extensive and direct evidence must precede any final 

 conclusions. 



Another study of variation in insects, social and otherwise, is that 



"Warren, E., "Some Statistical Observations on Termites, Mainly Based 

 on the Work of the Late Mr. G. D. Haviland," Biometrika, Vol. VI., pp. 329- 

 347, 1909. 



