650 Sir James Grichton- Browne [^Itiy 3, 



is to the right of the axis of the spire, so that if we imagine the shell 

 to be a spiral stakcase, we should in ascending it have the axis of the 

 spire to the left. We have here the shell of the common snail, Helix 

 aspersa (Fig. 33), which is so widely distributed over Europe, Asia, 

 Africa and iimerica, which is commonly dextral but in which occa- 

 sional instances are found of a sinistral turn ; kerry-handed shells. 



We have next the Fulgar carnica from Florida, which is commonly 

 sinistral with occasional dextral specimens (Fig. 34) ; and next the 

 Ampidromus inversus^ very rare, from Annam, which is a near 

 approach to an ambidextrous species, of which dextral and sinistral 

 specimens seem to be about equally numerous (Fig. 35). Some 

 genera are normally sinistral in the embryonic state, but afterwards 

 become dextral. 



Now no explanation can be offered of this dextral pre-eminence in 

 shells and these twists and turns in them, but this is certain that they 

 are not due to education or individual environment, for they are seen 

 already declared in the embryo. 



I need not remind you that plants as well as animals have dextral 

 and sinistral constitutions. Darwin reported many interesting ex- 

 amples of this in twining and climbing plants. He observed that 

 the hop travels slowly round its support to all points of the compass, 

 moving like the hands of a watch with the sun from right to left right, 

 and that the Geropegia gardnerii — an Asclepiadaceous plant — revolves 

 in a course the reverse of the hop and opposed to the sun (Fig. 36). 

 " A greater number of twiners," Darwin says, " revolve in a course 

 opposed to that of the sun than in the reversed course, and consequently 

 the majority ascend their supports from left to right (Fig. 37). 

 Occasionally, though rarely, plants of the same order turn in opposite 

 directions, but I have seen no instance of two species of the same 

 genus turning in opposite directions, and such cases must be rare." 

 But whatever the fixed habit of the species may be, occasional 

 individual departures from it are noted. Professor Assa Grey noticed 

 that in Thurga Occidentalism the twisting of the bark is very marked. 

 The twist is generally to the right of the observer, but in about a 

 hundred trunks four or five are seen to be twisted in the opposite 

 direction, and thus the number of reversals in this tree is exactly 

 in the same proportion as are left-handed reversals in human 

 beings. 



But we have not finished with dextral and sinistral manifestations 

 when we have reached the plants. We have to follow them down into 

 organic compounds. These substances are said to be optically active 

 when they produce rotation of the plane of a ray of polarised light which 

 passes through them, and the establishment of the connection between 

 optical activity and molecular asymmetry is not the least momentous 

 of the discoveries that we OAve to Pasteur. His later biological work 

 attracts more attention at the present time, but his researches of 1853 

 may yet prove to be even more fundamental and far-reaching in their 



