jan. 1899] EVIDENCE OF NATURAL SELECTION 39 



present in the other years, in crabs of a different size. The character 

 investigated is in a state of transformation during growth. Crabs of a 

 given size, say 12'1 mm., were narrower in 1895 than those of the 

 same size in 1893, but crabs of 1895, a little smaller, had the same 

 frontal breadth as those of 12 - 1 mm. in 1893. In other words, a given 

 mean frontal breadth was never wanting in any of the three years, but 

 was found associated with a different size of the crab. From this fact 

 alone it is impossible to base any argument concerning selection on the 

 variations observed and recorded in Professor Weldon's Address. The 

 variations investigated are variations of the mean frontal breadth in 

 proportion to carapace length, and it is not even suggested that any 

 particular variations were absent, or less frequent, less common, in one 

 year than another. It is merely stated, and may be accepted as a 

 fact, that each degree of the dimension w r as found in smaller crabs in 

 the later years. It does not matter whether we say that crabs of a 

 given size were found to be narrower in frontal breadth, or that crabs 

 of the same frontal breadth w r ere found to be of smaller size. In 

 consequence of the diminution of the frontal breadth with increase of 

 size both statements are equally true. 



Before proceeding to the explanation given of the cause of the 

 change, it may be noticed that in a footnote a previous hypothesis put 

 forward by Prof. Weldon concerning the selective destruction of varia- 

 tions in this character is now abandoned, and we are informed that it 

 is to be replaced by the results contained in the Presidential Address. 

 That previous hypothesis was that the narrowest as well as the broadest 

 fronted crabs were killed off, so that the range of variation in the older 

 and larger crabs was reduced. Of this previous conclusion a very eminent 

 Darwinian said at the time, in a letter to Nature, that its demonstration 

 "deserved to rank among the most remarkable achievements in connection 

 with the theory of evolution." It is evident now that this admiration was 

 premature ; the process was hypothetical, had never been demonstrated 

 at all, and the hypothesis is now abandoned in favour of a kind of 

 selection which is quite inconsistent with that formerly suggested. 



The measurements, to which I have referred above, all relate to 

 male crabs ; the change in females during the same time is said to be 

 less in amount though in the same direction. 



Prof. Weldon is confident that the change is due to a selective 

 destruction caused by increase in the amount of sediment in Plymouth 

 Sound. According to his argument the turbidity of the w r ater in the 

 Sound is increasing because the breakwater obstructs the scour of the 

 tide and removal of the silt, while china clay is brought down by the 

 rivers from Dartmoor, and the increasing population, dockyard, and 

 shipping imply the passage of more sewage and refuse into the Sound. 



In order to ascertain whether the broader fronted crabs were killed 

 by sediment, a number of specimens were put into a large vessel of 

 water in which fine china clay was placed and kept suspended by an 



