THE FOLLOWERS OF GEOFFROY 



in many of its organs the adult stage of various lower 

 animals. It is about this time that it possesses a tail. 



We note that Serres' theory of parallelism applies, strictly 

 speaking, only to organs, not to organisms, although he, too, 

 readily fell into the error of supposing that the organisation 

 of an embryo could be compared as a whole with the adult 

 organisation of an animal lower in the scale. Thus he wrote 

 in one of his later papers 1 " As our researches have made 

 clear, an animal high in the organic scale only reaches this 

 rank by passing through all the intermediate states which 

 separate it from the animals placed below it. Man only 

 becomes man after traversing transitional organisatory states 

 which assimilate him first to fish, then to reptiles, then to 

 birds and mammals." Serres was not altogether free from the 

 besetting sin of the transcendentalists hasty generalisation. 



The law of parallelism applied not only to Vertebrates 

 but also to Invertebrates. In a short paper 2 of 1824 Serres 

 attempted an explanation of the nervous system of Inverte- 

 brates. Invertebrates, he considered, lacked the cerebro- 

 spinal axis of Vertebrates, and their nervous system was the 

 homologue of the sympathetic system of Vertebrates. The 

 relation of the invertebrate to the vertebrate nervous system 

 being thus fixed, can the nervous system of Invertebrates be 

 reduced to one plan ? It does not seem possible to establish 

 a common plan for the adult nervous systems. But apply 

 the principle of parallelism, which has proved so valuable 

 within the limits of the vertebrate series. Taking insects as 

 the highest class, we find that there are three stages in the 

 development of their nervous system ; in the first the nervous 

 system is composed of two separate strands, in the second 

 the strands unite round the oesophagus, in the third they 

 unite also behind. Now in llnlla apcrta, stage (i) is 

 permanent ; in Clw t Doris, Aplysia, Tritotiia, Sepia, Jldi.v, 

 stage (2) is permanent, and in Unio stage (3). In fact, all 

 the varieties of the nervous system of molluscs fall into one 

 or other of these three classes. " It follows, then, that as 

 regards their nervous system, the Mollusca are more or less 

 advanced larvae of insects " (p. 380). The law of parallelism 



1 Ann. Sci. not. (2), ii., p. 248, 1834. 

 " Ann. Sci. //<//., iii., pp. 377-80, 1824. 



