440 PROCEEDINGS OP THE AMERICAN ACADEMY. 



V. Discussion of Results. 



Concerning the original purpose of this study, the determination of 

 sonic place-modes for the Fiddler Crab, it may lie said that a number of 

 modes have been given in Table [., which it is hoped will be of value in 

 the future as helps in the study of racial variation and of the formation 

 of Bpecies. As before remarked, those measurements which would seem 

 the most valuable for place-modes are : (a) the frontal breadth, (b) the 

 median length, " ' the right marginal length, and (<7) the left marginal 

 length. 



It remains to be asked, What answers do the results of this Btudy 

 enable us to give to the group of questions centring about the Great 

 Chela condition, with which we Bel out ? (1) What is the significance 

 of right- and left-handedness ? (2) What relation does the size of the 

 il chela bear to other dimensions ? < 3 ) What determines right- and 

 left handedness ? 



The chief significance of the gr< at chela, observation leads me to con- 

 clude, ia in its value (1) as a meaus of defence and offence, and (2) as a 

 means of burrowing. Bui granting that these are sullicient reasons for 

 its existence, we have still to ask why it is sometimes on the right Bide, 

 Bometimes on the left. The fact that approximately equal numbers ol 

 R ghtS and Lefts are found, seems to indicate that the great chela is not 

 determined by heredity, or, at least, not directly. For if this were the 

 case, the probability of an equal distribution between the two types 

 would be very Blight It is more likely, therefore, that we are dealing 

 with what is usually called (although improperly) a chance determina- 

 tion: that is, there are a number of variable factors, only partially 

 known at present, which throw the greatrchela-developing-tendencj now 

 ibis way. now that. Other instances of determination of this kind are 

 the determination of Bex and of the crossing of the optic nerves. IYo- 

 or < i. II. Parker, who has studied the crossing of the optic nerves 

 of (idle,, bm whose results have not yet been published, has found 

 thai many symmetrical Bshes are about equally divided between those 

 having the right nerve crossing above the left and those having it en 

 iic_ r below the left. Since ii is at present impossible to point out, in bucIi 

 any uniform cause or group of causes for the condition, we sa\ it is 

 a matter of chance. 



I ii answer to the second question, — What relation does the Bize of the 



it chela bear to the oilier dimensions ? _ it uiay be said, that the in 



urements which allow of com pan - I the two sides ol the body, namely, 



