42 J. T. CUNNINGHAM [januaky 



died in the experiments with clay and mud because they were younger 

 and weaker. In the same way the crabs that moulted in the bottles 

 possibly grew more than those in the sea, because they were kept in 

 warmer water and supplied with more food. Therefore they were, after the 

 moult, larger than those in the sea of the same relative frontal breadth. 



My suggestion is also in accordance with the fact stated by Professor 

 Weldon that there was less change in female crabs than in male. If 

 the difference of frontal breadth was the cause of selective destruction, 

 it is difficult to see why it should act on one sex more than on the 

 other. On the other hand, it is known that male crabs are larger when 

 adult than females, therefore they grow faster, and their growth would 

 be more affected by changes in the conditions of life in different years. 



Professor Weldon states, in reply to my letter in Nature, that he 

 has tested my suggestion and found that it is not in accordance with 

 the facts. Apparently he has examined the measurements of the 

 crabs which moulted in his experiment, and found that those which 

 increased most in size at the moult did not differ in frontal breadth 

 from the mean of those of the same size on the shore, more than those 

 which grew least. According to Professor Weldon's original statement, the 

 crabs that moulted were on the average broader than those of the same 

 size on the shore. According to my suggestion, the broadest individuals 

 should be those that grew most at the moult. This my opponent denies. 

 I have asked him to produce the figures, but he declines to do so 

 until all the figures relating to the matter are ready for publication. 



As a proof that in the experiments with mud or clay the youngest 

 crabs did not die first, he has instanced an experiment in which, out of 

 many crabs 10 to 15 mm. long, only four survived, all under 13 mm. 

 But he does not say that these four were the narrowest, as they ought 

 to have been on his own hypothesis. If they were, they may also have 

 been the oldest, or they may have been, if my second suggestion (see 

 below) is correct, those which had grown fastest and were therefore the 

 most vigorous. If they were not narrower than all the others which 

 died, the experiment does not support Professor Weldon's conclusions. 



Mr. Walter Garstang has challenged my suggestion in another 

 way. In a letter to Nature he contends that exuviation is essentially 

 connected with the process of growth in Crustacea, and that " in 

 assuming similarity of size in young shore crabs to indicate an equal 

 number of moults, Professor Weldon is quite in accord with our 

 present knowledge of the subject." It would take up too much space 

 to enter here upon a discussion of the question whether ecdysis is 

 essentially related to increase in size or not, but I may refer to certain 

 facts as to the relation between size and the number of moults. 



In 1884 the late Mr. George Brook published a paper on the 

 " Eate of Development of the Common Shore Crab " {Ann. and Mag. 

 Nat. Hist. vol. xiv. pp. 202-207). He recorded a number of observa- 

 tions and measurements of young crabs kept alive for various periods in 



