LIXXEAN SOCIBTY OV LOXDOX. 113 



researches. A second paper on the Correlated Variations in 

 Cranr/on appeared in 1893, and about the same time he became a 

 member of a committee of the Eoyal Society formed for the 

 purpos^e of " Conducting Statistical Enquiries into the Measnrable 

 Characteristics of Phmts and Animals." In connection with this 

 Committee, he began an investigation on the variability of the 

 breadths of tlie frontal carapace in the common shore-ci'ab, Car- 

 cinns manas, and this led to Avhat was probably the most brilliant 

 of his discoveries. He found that the mean frontal breadth of 

 crabs talceu from Plymouth Sound was diminishing at a relatively 

 rapid rate, and came to the concliisiou that this diminution was 

 due to the changed conditions of Plymouth Sound owing to the 

 great increase of the population of the three towns dischar<zing 

 their refuse into a bay nearly completely closed by a huge artilicial 

 breakwater, and also to the large quantities of china-clay discharged 

 into the Sound. To prove this, he undertook the most laborious 

 experiment of keeping a large number of crabs in water in which 

 a quantity of finely divided china-clay was kept sttspended. and he 

 found that the crabs with broader frontal carapaces, and therefore 

 wider res])iratory apertures, were choked by the china-clay and 

 died, while those ^^■ith narrower frontal carapaces survived. By a 

 still more laborious experiment he showed that in a number of 

 young crabs reared in clean water, and therefore protected from 

 the pernicious effect of mud and china-clay in sttspension, the 

 mean froittal breadth was raised above that of wild crabs of their 

 own size living in Plymouth Sound. This convincing experimeiit 

 formed the subject of AYeldon's brilliant Presidential Address to 

 the Zoological Section of the British Association at Bristol in 1898; 

 and it remains an almost unique instance of a clear demonstration 

 of the operation of Natural Selection in actual progress, while it 

 at the same time silenced those who objected that small and 

 apparently useless variations cottld not be preserved by ]Sattiral 

 Selection. What could seem more useless than a slight diminution 

 of the breadth of the carapace of a crab ? Tet a more extended 

 knowledge showed that it is in fact useful under certain conditions 

 and is preserved by Natural Selection. 



At IJniversity College "Weldon found a congenial spirit in 

 Professor Karl Pearson, who, like himself, had been fired with 

 enthusiasm for the statistical study of biological problems initiated 

 by Galton, and the mathematical professor was soon able to render 

 his colleague the most important assistance by the solution of the 

 problem of dealing with the asymmetrical distribution of variations. 

 They quickly became fast friends, and their cooperation has marked 

 an epoch in statistical biological research. 



In a short time Weldon, whose enthusiasm was contagious, had 

 a group of young and ardent naturalists working under his 

 direction, and in order to give expression to the new school of 

 zoological thought tints growing up tinder his guidance, he initiated 

 in 1901, in conjunction with Mr. Francis Galton, Professor Karl 



