YERKES. — VARIATION IN THE FIDDLER CRAB. 441 



the margin and the meropodite of the second leg, are larger on the 

 side of the great chela. Expressed in percentages, the relations are as 

 follows: Margin on chelar side in case of Rights, 5% greater than on 

 opposite side, — the same is true of the Lefts; meropodite of second leg 

 on chelar side in case of Rights, 6% greater than that of the opposite side ; 

 meropodite of second leg on chelar side in case of Lefts, 8% greater. The 

 fact that measurements are greater on the chelar side may be taken in 

 support of (a) the idea of chance determination, for any advantage of one 

 side of the body over the other would be likely to affect all organs. On 

 the other hand it may be held (b) that the development of the great chela 

 is itself the cause of the greater development of the other organs on the 

 same side. Or, combining these two views, it might be maintained 

 (c) that the causes which led to the development of the great chela were 

 operative also in case of the other organs, but that the development of 

 the great chela was an additional cause for the unusual size of the other 



.-> 



organs. 



As to the difference in size of the Rights and Lefts I can only say that 

 it is an interesting and surprising fact, for which I have found no expla- 

 nation. I at first interpreted the fact that the Lefts are the smaller and 

 the more variable as meaning that the Rights represented the stable 

 form ; and in support of this I had, as a result of the first summer's work 

 (see Table III.), evidence that there were more Rights than Lefts. But 

 this conclusion was not confirmed by the additional observations made 

 during the summer of 1900 (see Table IV.). It may be, however, that 

 the proportion of right- and left-handed animals varies in different col- 

 onies, and that during the interval between the collection of the animals 

 referred to in Table III. and those of Table IV. migration or some other 

 change had caused alteration in the relative frequency of Rights and 

 Lefts ; but this does not seem very probable. 



VI. Summary. 



1. The place-modes for twelve measurements on the carapace and 

 extremities of right- and left-handed Gelasimus pugilator arc given in 



Table I. 



2. The means of Table I. show that the right-handed animals are 

 larger and less variable than the left-handed. 



3 Right- and left handed animals occur in approximately equal 

 numbers. 



