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 like 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 
