MEASUREMENT OF NATURAL SELECTION 



527 



Fig. 2. Outline of the uppek sur- 

 face of the Carapace of the Shore 

 Crab, Garcinus mosnas. 



Fig. 2 will be recognized at once, even by the reader whose knowl- 

 edge of marine biology is limited to the menu-fauna of the city restau- 

 rant as the outline of the solid upper portion of the crab's body known 

 as the carapace. In measuring the frontal breadth 7 of Garcinus from 

 a particular spot of beach near 



the Marine Biological Laboratory . -- -—— 



at Plymouth, Weldon and Thomp- 

 son noticed a peculiar change 

 from year to year. For crabs of 

 the same length of carapace, 8 the 

 frontal breadth seemed to be de- 

 creasing. 



I have tried to make this clear 

 by a diagram. In Fig. 3 the indi- 

 viduals are classified into twenty- 

 five groups according to length of 

 carapace and the proportional 

 frontal breadth 9 for each class 

 represented for the three years by the position of the circles. 10 The 

 general slope of the connecting lines convinces one that the Plymouth 

 Sound crabs, as observed by Thompson and Weldon, were undergoing 

 a pronounced change in frontal breadth. 



The two reasonable hypotheses to account for this decrease are: 

 (1) A modification of the young individuals by the direct action of a 

 changing environment, (2) a decrease in the average frontal breadth 

 in the population due to elimination of the individuals with broader 

 frontal dimensions. 



A change in the environmental conditions of Plymouth Sound was 

 undoubtedly in progress during the time when Professor Weldon's ob- 

 servations were made. The streams bring into the sound large quantities 

 Measurable Characteristics of Plants and Animals), Proc. Roy. Soc. Lond., Vol. 

 XL VII., pp. 360-379, 1894. Also, W. F. E. Weldon, presidential address to the 

 Section of Zoology, British Association, Eeport of Bristol Meeting (1898), pp. 

 887-902, 1899. Interesting and valuable supplementary information concerning 

 Weldon's studies on selective elimination are to be found in Pearson's bio- 

 graphical memoir of Professor Weldon (see Biometrika, Vol. V., pp. 1-52, 

 pi. I.- V, 1906). 



7 The distance between the tips of the extra-orbital teeth, from the point 

 A to the point A' in the figure. 



s There is no way of knowing precisely how old an individual beast is ; if the 

 specimens for different series are sorted into classes of about the same length of 

 carapace, on a line from C to D, and if there is no reason to suspect any differ- 

 ences due to special environmental influences, dimensions of other parts of the 

 shell can be compared in different lots with reasonable confidence that animals 

 of about the same average age are being examined. 



9 The frontal breadth is expressed in thousandths of the carapace length. 



10 For 1898 the number of observations is not large enough for thoroughly 

 satisfactory determinations. 



