440 PROCEEDINGS OF THE AMERICAN ACADEMY, 



V. Discussion of Results. 



Concerning the original purpose of this study, the determination of 

 some place-modes for the Fiddler Crab, it may he said that a number of 

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

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

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

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

 median length, (c) the right marginal length, and (d) the left marginal 

 length. 



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

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

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

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

 great chela bear to other dimensions ? (3) What determines right- and 

 left-haudedness ? 



The chief significance of the great chela, observation leads me to con- 

 clude, is in its value (1) as a means of defence and offence, and (2) as a 

 means of burrowing. But granting that these are sufficient reasons for 

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

 sometimes on the left. The fact that approximately equal numbers of 

 Rights 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 slight. It is more likely, therefore, that we are dealing 

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

 tion ; that is, there ai'e a number of variable factors, only partially 

 known at present, whicli throw the great-chela-developing-tendency now 

 this way, now that. Oilier instances of determination of this kind are 

 the determination of sex and of the crossing of the optic nerves. Pro- 

 fessor G. H. Parker, who has studied the crossing of the optic nerves 

 of fishes, but whose results have not yet been published, has found 

 that many symmetrical fishes are about equally divided between those 

 bavins the risfht nerve crossing above the left and those having it cross- 

 ing below the left. Since it is at present impossible to point out, in sucli 

 cases, any uniform cause or group of causes for the condition, we say it is 

 a matter of chance. 



In answer to the second question, — What relation does the size of the 

 great chela bear to the other dimensions ? — it may be said, that the meas- 

 urements which allow of comparison of the two sides of the body, namely, 



