298 HEREDITY IN THE PROTOZOA 



environmental changes affect the kind of recom- 

 binations that occur at conjugation, it would be 

 almost impossible to show that one was not dealing 

 with such effects, rather than with mutation. 



It is not without interest in this connection to 

 call attention again to the results of selection in a 

 few of the higher plants where the basis of the 

 selected differences rests on plastids present in the 

 cytoplasm that multiply and transmit certain prop- 

 erties, independently of the hereditary complex 

 contained in the nucleus. Here selection also pro- 

 duces immediate results that may also be reversed 

 if not carried too far. Selection brings about 

 effects that are strikingly hke these described in 

 the Protozoa, yet the mechanism in these same 

 plants that gives in them Mendelian inheritance is 

 not affected by selection of such ''cytoplasmic" 

 differences. There is certainly no contradiction 

 here, but only two different processes each with its 

 own mechanism, both of which are equally entitled 

 to be called inheritance. Each of these kinds of 

 heredity may play an important role within its 

 own field, and in both the protozoa and the metazoa 

 both kinds of inheritance may take place side by 

 side. Such an interpretation leads then to the two 

 following considerations : 



First, it is thinkable at least, that selection in 

 the Metazoa might sometimes involve the cytoplasm 

 (and its inclusions) of the germ-cells themselves. 

 If so, selection might bring about changes compa- 

 rable to those described in the Protozoa. This might 



