ORIGINAL COMMUNICATIONS. 



Evolution and the Question of Chance. 



By F. W. Headley. 



Professor Weldon has shown that in shore-crabs the dimensions of 

 the carapace vary in accordance with the law of chance, and that the 

 variations group themselves nearly symmetrically on either side of the 

 mean. There is a large number of individuals slightly above the mean, 

 a large number slightly below. Measurements and observations of 

 other animals and of plants brought out similar results. If, therefore, 

 an increase in any particular dimension is of advantage, there are many 

 individuals in which this favourable variation is found, and, if it can be 

 proved that their slight superiority makes the difference between sur- 

 vival and non-survival, then, it is maintained, all is plain sailing, and 

 evolution pursues its way on strictly Darwinian lines. But is not 

 this too large an inference to draw from the facts ? Surely there still 

 remains a great deal in evolution to be explained. Indeed, the main 

 problem seems to me still to present itself without much alteration. 

 It is one thing to explain the gradual enlargement or diminution of a 

 particular organ, it is another to account for the various kinds of organs 

 and for the various faculties that animals have developed. To take an 

 instance, Prof. Weldon has proved that the frontal breadth of shore- 

 crabs varies, and by experiment he has obtained evidence suggesting that 

 the survival of an individual depends on its breadth in this region. 

 But this hardly helps us to solve the problem why crabs have developed 

 a chitinous exoskeleton instead of some other kind of cuticle. Are we 

 here also to appeal to the law of chance and say that an unlimited 

 number of kinds of external cuticle were available ; that thousands of 

 variations, each offering a different kind of integument, made their 

 appearance ; and that this particular kind of chitinous covering was 

 selected in the case of crabs ? It will be at once apparent that the 

 law of chance as explained by Prof. Weldon — the tendency of vari- 

 tions to fall on either side of a mean — has no application in a case 

 such as this when the question is one not of amount but of kind. It 

 is true that all variations are due to chance, in the sense that we cannot 

 25 — nat. sc. — vol. xiv. xo. 87. 357 



