DARWINISM DEFENDED. 161 



"I think, therefore, that Mr. Thompson's work, and my own, 

 have demonstrated two facts about these crabs ; the first is that 

 their mean frontal breadth is diminishing year by year at a measur- 

 able rate, which is more rapid in males than in females ; the second 

 is that this diminution in the frontal breadth occurs in the presence 

 of a material, namely, fine mud. which is increasing in amount, 

 and which can be shown experimentally to destroy broad-fronted 

 crabs at a greater rate than crabs with narrower frontal margins. 



"I see no shadow of reason for refusing to believe that the action 

 of mud upon the beach is the same as that in an experimental 

 aquarium; and if we believe this, I see no escape from the con- 

 clusion that we have here a case of Natural Selection acting with 

 great rapidity, because of the rapidity with which the conditions 

 of life are changing." 



These observations and conclusions of Professor Weldon have 

 been the subject of much discussion. The adverse criticism has, on 

 the whole, seemed to be successful in discrediting the case as an 

 example of any such clear-cut action of natural selection, as Weldon 

 seems to hold it to be. J. T. Cunningham (Natural Science, Vol. 

 XIV, pp. 38-45, 1899) concludes, after a critical analysis of the 

 work, that "Weldon's observations may be completely explained by 

 variations in the amount or rate of growth. The difference in 

 different years would be at once explained if the amount of change 

 in frontal breadth was constant for each moult, while the amount 

 of growth was variable. The fact is, that in 1893 crabs of a given 

 frontal breadth were larger than in 1895 and 1898; and I have 

 shown that the summer of 1893 was exceptionally fine and warm. 

 Either the warmth alone, or warmth and food together, very prob- 

 ably made the crabs grow more in that year for the same number 

 of moults. On this view the broad-fronted crabs died in the expe- 

 riments 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. 



'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 differ- 

 ences, 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 on 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 



