Professor Weldon's Evidence of the Operation of 

 Natural Selection. 



By J. T. Cunningham, M.A. 



In his Presidential Address before Section D of the British Association, 

 at its last meeting, Professor Weldon gave an account of his investiga- 

 tions of the variation of the shore crab in Plymouth Sound. The 

 Address is printed in the number of Nature for September 22, 1898. 

 The evidence it contains is considered by the author to prove, firstly, 

 that a certain change has taken place in one character of the animals 

 in question in the period of five years, and secondly, that the change 

 has been caused by a selective destruction of individuals, due to the 

 increasing turbidity of the water in which they live. These con- 

 clusions, if correct, would be of very great importance, and for this 

 reason it is necessary to examine and consider carefully the evidence 

 on which they are based. I have examined the evidence, and it does 

 not seem to me to furnish the proof required. I propose to state here 

 the reasons why I am unable to accept Professor Weldon's conclusions. 



He gives, in the first place, three determinations of the mean frontal 

 breadth of the crabs, expressed in terms of the carapace length taken 

 as 1000. In other words, he measures the frontal breadths of in- 

 dividual crabs, and expresses them as so many thousandths of the 

 length of the carapace. The three determinations are one for each of 

 the years 1893, 1895, and 1898. It is important to observe that the 

 mean frontal breadth in crabs collected at the same time varies very 

 rapidly with the length of the carapace. The mean was therefore 

 determined for every '2 mm. of carapace length in crabs from 10 to 

 15 mm. long. The larger the crabs, the less is the mean relative 

 frontal breadth. For example, in crabs 10'1 mm. long in 1893, it 

 was 816 thousandths of the length; in crabs 14'9 mm. long, it was 

 751 thousandths — a decrease of 65, omitting decimal fractions. 



For groups of the same carapace length the mean frontal breadth 

 was less in 1895 than in 1893, and less in 1898 than in 1895, but 

 the numbers measured in 1898 were admittedly very small. Here 

 arises the first objection to the argument. The dimension was less in 

 crabs of the same size, but every dimension observed in one year was 



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