Heredity in Protozoa 627 



individual are not the result of such modifications of the parent 

 cell as will cause a change in the process of reproduction such 

 as to proc^uce anew these characteristics; hence they ar not 

 inherited. This is true in both Protozoa and Metazoa. 



6 Thus the problem of how new inherited characters arise is 

 the same in Protozoa as in Metazoa. We may therefore work 

 on the general problem as readily in the one group as in the other, 

 and there is no reason why the principles reached in one group 

 should not apply equally to the other. Thus a new line of attack 

 on the problem is opened; in view of the rapid multiplication of 

 the Protozoa and the ready accessibility of their reproductive cells 

 both to environmental influences and to observation, this gives 

 some marked advantages. 



7 The search for the origin of new inherited characters (in both 

 Protozoa and Metazoa) resolves itself experimentally into a search 

 for agencies and processes which will permanently modify the 

 cell in such a way as to cause it to act differently in reproduction. 



8 When a given structural characteristic arises during the 

 reproductive processes so as to appear in a given generation, that 

 is not because the same structure was present in a preceding gen- 

 eration. Often indeed it was not present before; its origin is due 

 to some change in the constitution (chemical or structural .?) of 

 the preceding reproductive cell. Thus, the production of a spine 

 such as we see in Fig. 4 is evidently due to a spot of weakness at a 

 certain point in the cell body, causing a protrusion during fission. 

 Such a structure might result from the localized presence some- 

 where in the cell body of a certain chemical compound, which 

 would react at a certain stage with some other substance, thus 

 producing a spot of weakness, where a spine would be protruded. 

 So, the appearance of the new anterior setae in the posterior prod- 

 uct of division in the Hypotricha (Fig. 21) is evidently due in 

 some way to the constitution of the cell. 



9 Thus, then, the cause of the appearance of a certain struc- 

 ture in a certain generation is some other peculiarity of the cell 

 producing it; some chemical pecuharity, for example. We may 

 generalize this by saying that the appearance in the progeny of 

 « certain structure b is due to the existence in the mother cell of a 

 quite different condition a. 



