348 Tl^^ Theory of Natural Selection 



Measuring Selection 



During the nineties of the last century a breakwater 

 was built near the mouth of the Plymouth River in England. 

 At certain points in the sound the flow of the stream was 

 slowed up, so that there was an increase in the amount of 

 fine clay and mud settling in some regions. There followed 

 an increased mortality among the crabs. Professor W. F. 

 R. Weldon, a confirmed Darwinist, undertook to find what 

 differences were to be observed between the crabs that were 

 killed and those that survived. He made hundreds of meas- 

 urements. The killed crabs were slightly broader than those 

 that survived. This difference showed a consistent tendency. 

 Moreover, as measurements were continued year after year, 

 there was observed the tendency for the average width of 

 the surviving crabs to diminish. 



Similar measurements were repeated on crabs placed in 

 an aquarium with fine clay in suspension. Dr. Weldon 

 drew two conclusions from these studies. First, the average 

 frontal breadth of the crabs was diminishing year after year 

 at a measurable rate, which was more rapid in the males 

 than in the females. Second, this diminution of the average 

 frontal breadth was taking place in the presence of a ma- 

 terial which was increasing in amount and which could be 

 shown experimentally to destroy the broad crabs more rap- 

 idly than the narrow ones. 



Francis Galton was the first to attempt the application 

 of mathematics to biological problems. He had proposed 

 the statistical comparison of groups of plants and animals 

 in which selection could be observed. His methods were 

 later extended by Karl Pearson, under whose leadership 

 many such studies were carried on. Pearson collected meas- 

 urements over long periods of time, for many species and for 

 many characters. He compared the mean magnitude of 

 characteristics in organisms that died with the corresponding 

 measurements of those that had survived. He compared 

 the measurements of a given trait in the whole population 



