106 THE EVOLUTION THEORY 



the case of the Pacfurids it must not be hard because it could not 

 then be readily squeezed into the hard-walled and narrow recesses 

 of the Gasteropod shell ; in this case there has therefore been positive 

 selection. I have not yet referred to the fact that the chitinous 

 covering is certainly not living, though it is not exactly dead ; it 

 is a secretion of the epidermic cells, not a tissue, and we cannot 

 suppose that there are any nerve-endings in it. It is almost super- 

 fluous to say that the fact that the skin is cast is in itself enough to 

 make such an assumption untenable, for the whole of the assumed 

 delicate nervous network would be shed at every moult and torn 

 away from the nerves which lead to it. As far as my knowledge 

 goes, nothing of this kind occurs anywhere in the whole range of 

 the animal kingdom. 



Even if we assume, for the benefit of Zehnder's hypothesis, that 

 although there are no nerves in the chitin itself yet irritations 

 affecting the chitinous coat may be transmitted through it to the 

 delicate nerve-endings lying beneath it, this should take place in 

 a greater degree at the thin places of the skeleton than at the thick 

 parts I But this interpretation is again fallacious, for we see that 

 the tactile organs of Arthropods always break through the chitinous 

 cuticle and protrude beyond it in the form of setae. 



Of the many other opponents of my views in regard to the 

 transmissibility of acquired functional modifications, I need only 

 deal in detail with Oscar Hertwig. 



He seeks for direct proofs of an inheritance of acquired characters, 

 and believes that he has found these in the hereditary transmission 

 of acquired immunity from certain diseases. He reminds us of 

 Ehrlich's well-known experiments on mice with ricin and abrin. 



Even small doses of these two poisons kill mice, but they are 

 tolerated in very minute doses, and if their administration be con- 

 tinued for some time in such minute doses, the animals gradually 

 acquire a high degree of insensitiveness to these poisons ; they 

 become immune to ricin and abrin. 



This imnmnity is transmitted from mother to young, but it only 

 lasts for a short time, about six to eight weeks after birth. Yet 

 this is regarded by Hertwig as an illustration of the transmission 

 of an acquired character, as an acquired modification of the cells 

 of the body, for he explains the immunity on the assumption that 

 all the cells of the body undergo a particular variation due to the 

 influence of the poison, and are thus, to a certain extent, modified 

 in their nature, and that the ovum also undergoes this variation 

 and transmits it to the young animal. The immunization might 



