340 ACTION OF NATURAL SELECTION 



hence ought to be made, as Professor Weldon him- 

 self well recognises, in order to see whether the de- 

 structive process is still continuing. If this is the case, 

 and if crabs measured in, say, 1903 and 1908 show a 

 further diminution of frontal breadth, then the evi- 

 dence in favour of selection would amount to a very 

 high degree of probability indeed. Owing to the 

 changing relation of its parts with growth, the crab is 

 a somewhat unsatisfactory organism to work with, and, 

 indeed, the apparent change observed between 1893 

 and 1898 might possibly, though not probably, owe its 

 origin to quite another cause than Selection. For in- 

 stance, the conditions of environment such as tempera- 

 ture, nutrition, and purity of the water may have acted 

 directly on the crabs so as to retard their growth. 

 Now Professor Weldon assumes that all crabs of, say, 

 12 mm. length are approximately the same age, but ob- 

 viously this need not be so from year to year. Under 

 less favourable conditions, the moulting may have gone 

 on as usual, but the rate of growth have been reduced. 

 Now we have seen that the frontal breadth diminishes 

 very rapidly with growth, and hence it might happen 

 that the narrower fronted 12 mm. crabs of 1898 are 

 narrower simply because they are older than were the 

 more favourably situated 12 mm. crabs of 1893. Mr. 

 J. T. Cunningham * has pointed out that in 1893 the 

 temperature of the Channel waters was abnormally 

 high, and he considers that this produced a more rapid 

 growth of the crabs, and hence, for a given size of crab, 

 an apparent increase of frontal breadth. However, 

 Professor Weldon f does not believe that the tempera^ 

 * Nature, vol. Iviii. p. 593. \Ibid., p. 595. 



