SESSION II. DISCUSSION 



177 



the Prosobranchia (Orders Archeogastropoda and Mesogastropoda and sub-order Steno- 

 glossa) which inhabit ail warm seas. Among these the dextrotropic form predominates, 

 the shell being twisted Hke a right-handed screw [15]. The laevotropic form is extremely 

 rare, representing no more than o- 01-0-0005% of individuals. The preponderance of 

 the dextrotropic form in molluscs would seem to have become more widespread in 

 relatively recent times. Thus, shells of molluscs of the species Fusus antiquus dug up 

 from lower Permian formations and the protoconchs of some molluscs are of the laevo- 

 tropic configuration, while contemporary forms are of the dextrotropic configuration. 



Bearing in mind the special conditions of the equatorial zone, which enable more 

 ancient forms of organism to be preserved, we should remember that the optical activity 

 of petroleum is undoubtedly of organic origin. In this connection it is worth calling 

 attention to the fact that metamorphosed petroleums are distributed everywhere, the 

 exception being just the laevorotatory petroleums of the equatorial belt, from Java, 

 Borneo and South America. 



The salient evidence concerning the geographical distribution of dextro and laevo 

 forms of organism (and also petroleums) derived from the scanty literature is collected 

 in Table i. The signs D and L denote right-handed and left-handed (or, in the case of 

 petroleums, dextrorotatory and laevorotatory) forms respectively. 



Table i 

 Geographical distribution of dissymmetric forms 



As may be seen from the table the inverse, or less widely distributed organisms (mol- 

 luscs or bacteria) which have retained their form since ancient times, are to be found in 

 those places where radiation is even now considerable, namely, in the equatorial zone 

 and in mountainous regions. In northern regions the inverse forms are extremely rare. 

 How can this be explained? All the facts which have been enumerated are hard to under- 

 stand if we start from the natural assumption that a great part was played in the setting 

 up of the dissymmetric body-form of organisms by the right-handed component of 

 circularly polarized light, acting incessantly throughout the miUions of years of the 

 evolution of organisms, to produce, predominantly, one dissymmetric form. 



In acting as a selective factor the right-handed component of circularly polarized light 

 may have had a double action. When there was excessive solar radiation in the Archaeozoic 

 and Palaeozoic it permitted the survival of those forms of very simple organism which 

 were less able to sustain the effects of the solar radiation which, at that period was very 

 hard. Under these conditions the form of an organism which was more susceptible to 

 the effects of radiation could not extend its habitat and must have either been destroyed 

 or, as L. S. Berg [17] maintains, have hidden itself in the deep layers of the soil or water. 

 As the intensity of the irradiation decreased at the end of the Palaeozoic, the form of the 

 organism which was able to extend its range and develop was just the very one on which 

 the powerful radiation had earlier had a destructive effect owing to its relatively great 

 susceptibility. 



