1 98 THE VARIATION OF ANIMALS IN NATURE 



and (c) as to the interpretation of the measurements. It is 

 necessary to make it clear that there is a definite differential 

 (heterogonic) growth-effect involved in the relation of M.F.W. 

 to carapace length. M.F.W. decreases in proportion to the total 

 length of the carapace. 



(a) (i) Weldon did not show that the amount of silt had 



increased in the period under consideration ; he 

 merely assumed that it had. 

 (ii) He did not take into consideration the exceptional 

 climatic conditions of 1893, which may have had 

 a marked effect on growth and in consequence on 

 measurements correlated with absolute size. 



(b) Weldon assumed that M.F.W. would affect the filtration 

 of water in the gill-chamber, the narrower frontal breadth 

 forming a better filter. It seems very strange that the actual 

 entrance of the gill-chamber itself was not measured. Weldon 

 makes no attempt to show that there is any relation between 

 the two dimensions. As Cunningham (I.e. p. 193) points out, 

 ' the exclusion of particles of silt must depend on the absolute 

 size of the entrance to the gill-chamber, not on the proportion 

 which that size bears to the body-length.' 



(c) (i) Vernon (I.e. p. 340) objects that to take length for 



age is a dangerous procedure. Silt may retard 

 growth. 12-mm. crabs of 1898 may have a 

 narrower M.F.W. because they are older than those 

 of 1893. This objection assumes, of course, that 

 M.F.W. may be determined by age and not by 

 size. 

 (ii) A more serious objection is that of Cunningham 

 (I.e. p. 192). He points out that 'it would 

 follow from Weldon's argument that the pro- 

 portional frontal breadths which were fatal to 

 small crabs of a given carapace length, permitted 

 the survival of others which were only y mm. 

 shorter.' Thus, if in 1895 the M.F.W. of size-class 

 14-5 mm. has been reduced by selection from 

 762-00 to 754-45 in 1895, how is it that we find 

 all those less than 13-7 mm. size surviving in 

 which the M.F.W. is over 762-00 ? 



