FRANCIS H. HERKICK. 



conditions of the problem seem to be very simple, and can at 

 least be stated with a certain degree of exactness. 



The segments of the limb mainly affected (Figs, i and 2) are 

 the fifth and sixth (P and Cp, Figs, i and 2) which move on 

 hinge joints. Each has a hard tubular shell, and lodges two 

 muscles, a flexor and extensor, the fibers of which originate on 

 the inner surface of the shell, and are inserted on tendons which 

 engage with the next or distal segment, the mobility of the limb 

 being further secured by soft interarticular membranes, and a 

 series of hinge or peg-and-socket joints, set in different planes 

 (Figs, i and 2). When the flexor muscle, let us say, of the car- 

 pus or fifth joint contracts, the huge claw moves on its hinge 

 and is drawn upward and inward, as a door might be opened by 

 means of cords, worked at a distance : at the same time the 

 fibers react on the inner surface of the tubular shell, but the 

 hinge-joint is fixed, and no conceivable contraction of such fibers 

 can convert a straight pull into a twist. There is no room for 

 the shifting or migration of the muscles, but this would not affect 

 the conditions one way or another. Torsion has not occurred 

 as a result of the normal movements of the limb. 



It is to be noticed that the rotation of the claws occurs in larval 

 life before the chelai attain their full development, but if in the 

 course of the evolution of these forms the increasing weight of 

 the claws could have had any effect upon their ultimate position, 

 it should have turned them in the opposite direction, because they 

 lean outward in the larva. The theory that the effects of strain 

 or use are inherited is therefore incompetent to account for the 

 modification which exists. 



In the present case it seems equally impossible to apply the 

 selection theory of Darwin, and it would tax the imaginative 

 faculty to determine wherein the chela of a crayfish was better 

 adapted for prehensile purposes than the claws of hundreds of 

 prawns, or how the toothed claw of the lobster excelled in this 

 respect the pincers of the blue crab, Callinectes, or as a weapon 

 of offence, the slashing, sabre-like, out-turned dactyles of AlpJieus 

 heterochelis, and of many another species of this large genus. I 

 refer only to the power and ease of seizing, and not to the muscu- 

 lar force or to the armature of the claws itself, which is another 



