290 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



off. The experiments also suggest that it is the chemical character of the 

 surrounding medium which is responsible for the changes taking place in the 

 constitution of the individual. It is known that protozoa can become adapted 

 to certain toxic substances and to higher temperatures; here likewise, the 

 alterations of the individuals may be reversible. Do we have to deal in these 

 cases with functional, phenotypic changes in these unicellular organisms or 

 in certain parts of them, or do we have to deal with changes in their genetic 

 constitution? 



The observations of Reynolds in Arcella bear some resemblance to reactions 

 noted in certain of the higher vertebrates, by means of which the latter are 

 able to distinguish not only between species, but also between individuals or 

 related groups of individuals within the same species ; as an instance, we may 

 cite the recognition by dogs of individual scents. In the latter phenomenon 

 true individuality differentials are not involved, but the characteristics used 

 for differentiation between individuals are localized in certain organs and 

 tissues. 



However, one important feature these reactions in protozoa have in com- 

 mon with the reactions due to individuality differentials in higher organisms, 

 namely a graded differentiation between different organisms in accordance 

 with their relationship ; this similarity may be taken as an indication that also 

 in certain protozoa differences exist in the constitution of individuals as 

 well as of different species. We may then provisionally hold that the reactions 

 which we have studied in this chapter are due to substances analogous to but 

 not identical with organismal differentials, substances in particular analogous 

 to individuality differentials. It may furthermore be assumed that in protozoa, 

 too, a differentiation of cytoplasmic constituents and also of genetic substances 

 has taken place in the course of evolution, which has made the production of 

 such substances and the manifestation of these mechanisms possible. It seems 

 that a finely adjusted constitution of the surface layer of these protozoa has 

 made possible the individuality, race and species reactions, which take place 

 when two individuals or parts of individuals come in contact with each other. 

 However, in addition to contact actions, the organisms seem to exert upon 

 each other also some distance actions. Thus, according to Reynolds, Arcella 

 moves in the direction towards detached autogenous or syngenesious pieces of 

 Arcella, but it is not attracted by fragments of individuals belonging to the 

 species Difflugia. The distance reactions and the substances on which they 

 depend are apparently not so finely graded as the contact reactions. We may 

 perhaps interpret, in this sense, the observation that parts of an Arcella, which 

 in the course of generations has lost an autogenous or syngenesious contact 

 reaction, the latter having been changed into a homoiogenous reaction, may 

 still retain an autogenous reaction towards Arcella if it is not in direct con- 

 tact with it. While the contact action may depend upon sessile or only slightly 

 diffusible substances, the distance reactions in all probability are mediated by 

 diffusible substances. 



But, not in all unicellular organisms have such finely graded reactions, 

 indicating the relationship between individuals, been observed. Thus in As- 



