ON VARIATIONS. 337 



recently, and this is not to be wondered at, considering 

 the numerous and extended observations it, as a rule, 

 entails. In the case of the small shore crab, Carcinus 

 mocnas, however, Professor Weldon * has succeeded in 

 overcoming most of the inevitable difficulties and pit- 

 falls, and has obtained evidence which, though at 

 present not absolutely convincing, yet has a very high 

 degree of probability of truth. In 1893 Mr. H. 

 Thompson carefully determined the relation of the 

 mean frontal breadth to the carapace length in male 

 crabs collected at a particular patch of beach in 

 Plymouth Sound. The mean breadth was found to 

 vary very rapidly with the length of the crab, hence its 

 value was determined separately in small groups of 

 crabs, differing from each other by not more than 

 .2 mm. Twenty-five such groups, for crabs between 

 10 and 15 mm. long, were measured in respect of fron- 

 tal breadth. A similar series of measurements was 

 carried out by Thompson on crabs collected at the same 

 spot in 1895, and another by Weldon on crabs collected 

 in 1898. On comparing the three series of data thus 

 obtained, it was evident that the mean breadth of crabs 

 of a given carapace length had steadily decreased. For 

 instance, in crabs with a carapace length of 11.5 mm., 

 the frontal breadth had a percentage length of 79.72 

 in 1893, 78.88 in 1895, and 78.40 in 1898. Again, in 

 14 mm. crabs, it had a length of 76.26 in 1893, 75.44 

 in 1895, and 74.44 in 1898. 



It would seem, therefore, that the frontal breadth of 

 these crabs is diminishing, year by year, at a very rapid 

 rate. This Professor Weldon attributes to a selective 

 * Report of Brit. Assn., 1898, p. 887. 



