I08 FOOT-NOTES TO EVOLUTION. 



serve to simplify some of the intricate problems to be 

 explained, but surely no one believes that development 

 can ever be referred entirely to such factors. The fact 

 is that determinism, which is the most fundamental 

 characteristic of inheritance, is manifested at every step 

 of development, and there is certainly no escape from 

 the conclusion that this determinism depends upon pro- 

 toplasmic structure, and that this structure it is which is 

 transmitted from generation to generation, and which 

 forms the physical basis of inheritance. 



All really inherited characters must, therefore, be 

 represented in the structure of the germinal protoplasm, 

 and must consequently be present from the beginning of 

 development. " We must consider it as a law derivable 

 from the causality principle," says Hatschek,* "that in 

 the phylogenetic alterations of an animal form the end 

 stages are not alone altered, but the entire series from 

 the egg cell to the end stage. Every alteration of an 

 end stage or addition of a new one must be caused by 

 an alteration of the egg cell itself." Nageli f has ex- 

 pressed a similar view in the following famous sentence: 

 " Egg cells must contain all the essential characteristics 

 of the species as perfectly as do adult organisms, and 

 hence they must differ from one another no less as egg 

 cells than in the fully developed state. The species is 

 contained in the egg of the hen as completely as in the 

 hen, and the hen's egg differs as much from the frog's 

 egg as the hen from the frog." 



4. The remarkable tenacity of inheritance, as shown 

 especially in reversions and the preservation of useless 

 and embryonic characters through many hundreds or 



* Berthold Hatschek. Ueber die Entwickelungsgeschichte von 

 Toredo. Arb. Zool. Inst., Wien, 1880. 



\ Nageli. Mechanisch-physiologische Theorie der Abstam- 

 mungslehre, 1884. 



