1899] EVIDENCE OF NATURAL SELECTION 45 



crabs would be broader than those of the same size on the shore. 

 Whatever the influences of conditions on growth may be, I feel convinced 

 that the facts observed by Professor Weldon are due to such influence, 

 and for the reasons I have given, not to selective destruction. I have 

 shown that either of my two hypotheses would explain the facts better 

 than the selective destruction which Professor Weldon believes to be 

 the cause. 



In 1895 Professor Weldon stated that his statistical results could 

 not be considered to be established until the law of growth of the 

 crabs had been experimentally investigated. Yet he now publishes 

 with equal confidence quite different results from statistical data, 

 although in the meantime he has produced no evidence whatever 

 concerning the relation between variations of growth and change in 

 the relative frontal breadth. 



In reply to my letter in Nature Professor Weldon urged that, if 

 my suggestions were correct, crabs gathered in January would be 

 narrower in the frontal region than those of the same size gathered in 

 August, and he had found that those gathered last January were not 

 narrower than those gathered last August. But such a result does 

 not follow from my view. According to Mr. Brook's experiments 

 crabs above 10 mm. in length would be at least a year old, and 

 therefore had been under the influence of all seasons of the year, and 

 shore crabs hatch their eggs from February or March till late in 

 summer. Therefore crabs taken in January have not necessarily grown 

 under more unfavourable conditions. On the other hand, the observa- 

 tion of Professor Weldon that the crabs taken in January were not 

 narrower than those taken in August shows that there had not been a 

 continuous diminution in frontal breadth from August 1893 to August 

 1898, as he stated in his Address. 



Whether either of my suggestions in explanation of the observed 

 facts be correct or not, I think I have shown that the evidence on 

 which Professor Weldon bases his conclusions is quite inadequate to 

 establish them. The change described is not, if terms are used 

 correctly, a change in the character of the species, but merely a change 

 in the rate of development. The variations investigated are not 

 individual differences, since each individual in the course of its growth 

 passes through each one of the variations in its own person. It has 

 not been shown that the change has gone 011 continuously for five 

 years, or that it has taken place only in waters where there is much 

 mud. If tadpoles of the same size were found to have shorter tails in 

 one year than in another, few biologists would draw the conclusion that 

 the result was clue to the selective destruction of those with the longest 

 tails. The more probable explanation would be that those with the 

 shorter tails were in a more advanced stage of their metamorphosis. 



1 Moerae Terrace, Penzance. 



