116 ASYMMETRIC ANALYi^hS 



SECTION II 



ANALYSIS OF THE EVOLUTION OF THE 

 NERVOUS SYSTEM 



1. Stereo-coefficients of Action of the Optical Isomers 

 of Nicotine in the Phijlogenetic Series. Since the two 

 optical isomers of nicotine exert their killing action by 

 the same mechanism but with a different strength one 

 can, by measuring this difference of potency in various 

 animals, study the properties of the specific receptive 

 substance in different species. In higher animals, as has 

 been already recorded above, there is some specific sen- 

 sitive substance which is affected to dift'erent degrees by 

 the toxic action of the dextro and the laevo isomers of 

 nicotine. Protozoa do not possess, as some observations 

 have shown, such a sensitive substance, and the dextro 

 and laevo isomers of nicotine are for them equally toxic. 

 The question arises of the nature of this specific sub- 

 stance and of the stage of evolution at which it first 

 appears. 



Greenwood, as early as 1890, carried out an extensive 

 comparative investigation on the action of connnon laevo- 

 rotatory nicotine on invertebrates, attempting to establish 

 a parallelism between the toxic eifect of this alkaloid 

 which affects, as is known, the nervous system of ani- 

 mals, and the evolution of the nervous system. On the 

 basis of purely qualitative observations he reached the 

 conclusion that ''the toxic effect of nicotine on any or- 

 ganism is determined mainly by the degree of develop- 

 ment of the nervous system. Thus for Amoeba the sub- 

 stance cannot be regarded as exciting or paralysing ; it is 

 rather inimical to continued healthy life. As soon as any 

 structural complexity is reached, the action of nicotine 

 is discriminating in such a fashion that the nervous ac- 

 tions which are the expression of automatism, that is, 

 which imply coordination of impulses, are stopped first. 

 This is seen dimly in Hydra, and it is more pronunced 

 among the medusae. When structural development goes 



