342 ORGANIC GROWTH 



cells, which are connected with the ectoderm, but which have 

 come to lie beneath it. 



Thus we have in the Medusae a laminar central nervous 

 system extending over the body, whose cells are commencing 

 to concentrate at spots particularly suitable for communication 

 with the external world, and there to form definite brains. 

 In the higher Metazoa, in Vermes, Mollusca, Arthropoda, 

 and Yertebrata, these brains, or ganglia, are completely 

 formed, but their embryonic condition still indicates the 

 epiblast as their original place of origin. 



But in the former case the first appearance of a morpho- 

 logically recognisable nerve-system in the animal series, its 

 formation from the ectoderm, can only be explained as the 

 effect of the constant action of external influences upon the 

 organism, and by the inheritance of this effect by the 

 inheritance of acquired characters with the aid of selection. 



Accidental variability of the germ-plasm as a determining 

 cause seems here also completely excluded. 



A more detailed account of the researches above mentioned 

 on the nervous system of the Medusse, and especially of 

 my experiments upon the subject, is contained in the address, 

 " On the Idea of the Individual in the Animal Kingdom," 

 which forms the Appendix to this work ; for a full discussion 

 I must refer to my original papers. 



Only one point further I must draw attention to : the 

 nerve-cells in the Ctenophora and Scyphomedusse are so far 

 from being morphologically differentiated and recognisable, 

 that I have been accused of mistaking connective-tissue-cells 

 for nerve-cells. It is, however, obviously, on my view of the 

 matter, necessary that nerve-cells should at the commence- 

 ment of their evolution be similar to other cells. It is a 

 known fact that even in higher animals the nerve-cells in tl^ 

 embryonic condition cannot be distinguished from other 

 embryonic cells. Only function could impress upon nervous, 



