292 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



the organisms belonging to these groups differ also in other characteristics 

 and represent, therefore, three physiologically distinct varieties. Certain 

 races belonging to variety 2 produce toxic substances, which may be the 

 same or different in different races. One of these substances is strongly 

 toxic for all the races belonging to varieties 1 and 3, and only weakly toxic 

 for races belonging to their own variety, namely, 2. This toxic substance 

 affects, markedly, also certain other species of Paramaecium. Similarly, the 

 toxic substance produced by another race of variety 2 exerts marked dele- 

 terious effects on all the races of varieties 1 and 3, but is very injurious only 

 for certain races of its own variety, whereas others are more resistant or 

 completely immune. This toxic substance also acts strongly on certain strange 

 species, but not on others. In these cases we have evidently to deal with a 

 reaction type, which we have designated as "specific adaptation," the specific 

 adaptation depending in this instance presumably upon the specific chemi- 

 cal nature of the toxic substances and of certain receptor substances in 

 the different varieties, which insures a decreased toxicity of a substance se- 

 creted by Paramaecium aurelia for nearly related organisms. Sonneborn made 

 it probable that Mendelian rules of inheritance are applicable in the trans- 

 mission of the characteristics determining reaction types to successive gen- 

 erations of the various races. The determining factors were contained in the 

 micronucleus, but exerted their influence by way of the macronucleus and 

 cytoplasm. However, these genetic factors were accessible to environmental 

 conditions, and, in particular, the temperature prevailing at the time when 

 the macronucleus is formed from the micronucleus could influence the 

 proportion of individuals belonging to certain types. After this sensitive 

 period has passed the mating type is inherited by all subsequent macronuclei 

 produced at later fissions without further interference by the temperature. In 

 other organisms studied by Moevus, light is able to suppress the mating 

 reaction and in certain cases the mating types seem to be determined entirely 

 by environmental factors. 



We see, then, that in interactions of certain protozoa, comparable to ferti- 

 lization reactions in higher organisms, factors play a role which tend to 

 prevent fertilization with nearly related organisms and favor fertilization 

 with selected, more distant groups of the same species, and that differences in 

 reactions may have to be attributed to genetic differences in the constitution of 

 clones ; there are, furthermore, indications that also in these protozoa changes 

 in the constitution of the genetic substance may take place and thus increase 

 the diversification of various stocks. In contradistinction to these genetic dif- 

 ferences between various stocks of Paramaecia just discussed, certain struc- 

 tural abnormalities, which may be found in some individual Paramaecia raised 

 in cultures, do not seem to affect the mating reactions, inasmuch as such 

 abnormal individuals can be made to fuse with normal ones under the same 

 conditions as can other normal individuals. 



Is it correct to attribute these reactions between different groups of Para- 

 maecium to mechanisms comparable to those occurring in higher organisms 

 under the influence of organismal differentials? There are some apparent 



