1907.] on Dexterity and the Bend Sinister. 649 



she thought that the flat fish were made so bj being beaten out or 

 passed through rollers by the fishmonger, simply for table purposes 

 like crimped cod. The members of the Royal Institution know better, 

 but I am not sure they all know that certain species and genera of the 

 Pleuronectidffi are dextral or sinistral : that is to say, that in one 

 species it is invariably the right side of the fish that is uppermost, 

 coloured and carrying the orbits, and the left side that is downwards 

 and colourless ; while in another species the reverse of this holds good. 

 I have said " invariably," but that is hardly correct, for the remarkable 

 fact is that in each species there are occasional rare exceptions to the 

 specific rule, a few sinistral fish occurring in a dextral species, just as 

 there are a few left-handed individuals amongst right-handed human 

 beings. 



I am not sure, too, that all Members of the Royal Institution 

 know that the flat fish make a perfectly straight start in life. The 

 very young are transparent and symmetrical throughout, with an 

 eye on each side, and swim in a vertical position like other fishes ; 

 and it is as they grow that the skull becomes twisted, and that the 

 eye of one side moves round by degrees to the other side and becomes 

 the upper eye. 



This transformation in the flat fishes cannot be attributed to 

 prejudiced mothers or siUy nurses, or hide-bound schoolmasters or 

 acquired habits ; and the fact that it occurs at a particular stage of 

 growth disposes, I think, of the argument that dextral pre-eminence 

 in the human being must be induced by education, because the baby 

 for the first eight or nine months of its life uses both of its fore-limbs 

 equally. All dextral and sinistral forms and tendencies emerge at 

 different stages in the growth of the animal, in accordance with 

 specific predestination ; and some of them, as the sinistral establish- 

 ment of voluntary speech in the brain, com.e late. 



Next let us take the Crustacea, and with the lobster all of us have 

 an intimate and agreeable acquaintance, and all of us have, I doubt 

 not, observed that notwithstanding its striking symmetry, its two claws, 

 or forceps, which are to it hands and jaws, sometimes vary in size. 

 But more important than that, they differ essentially in structure and 

 in function. I show you a photograph of the claws of Homarus 

 gammarus, the common lobster, and you will observe a marked differ- 

 ence in the chelae (Fig. 31). On the right side you have the sharp 

 tubercles, or teeth we may call them, adapted for the cutting and 

 tearing of food, and on the left side the blunt tubercles, adapted to the 

 crushing and grinding of it. You will see this better in a photograph 

 of the chelae of the American lobster, Homarus Americanus (Fig. 32). 

 Now let us for one moment turn to the shells, in the spiral forms 

 of which dextral or sinistral tendencies are conspicuous. By far the 

 greater number of univalve spiral shells are, Uke human beings, dextral, 

 but in nearly every genus a few sinistral monsters occur. If held with 

 the spire pointing upwards and the aperture downwards, the aperture 



