332 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



The degree to which this type of specificity between different members of the 

 same group exists varies according to the difference in the genetic constitution 

 of different individuals of such groups, as well as according to the phylo- 

 genetic and ontogenetic stages of development which these organisms have 

 reached. It is very difficult, or perhaps impossible to eliminate entirely these 

 differences between different members of the same group through long-con- 

 tinued, close inbreeding, which starts with two different individuals; but by 

 these means these differences can at least be very much mitigated in the course 

 of time, the length of which varies in the case of different species. These 

 specificities have reached their, most fargoing development in the highest 

 organisms, which otherwise are the most rigid and the least modifiable as far 

 as their tissue and organ constitutions and the interrelation between the latter 

 are concerned. They are at the lowest stage of development in the phylo- 

 genetically and ontogenetically most primitive organisms, especially as far as 

 the manifestation of these reactions is concerned. The mechanisms which 

 underlie such specificities under different conditions vary in different stages 

 of the phylogenetic and ontogenetic evolution. In the course of the former, 

 these specificities, or at least their manifestations, are newly created step by 

 step, while in ontogenetic development they are present in the form of pre- 

 cursor substances and mechanisms, which in the end lead to the complete 

 formation of systems of the individuality differentials of the higher organ- 

 isms. The substances and the mechanisms on which the maintenance of these 

 equilibria depends may accordingly vary to a certain extent under different 

 conditions. 



Thus, in some free-living single cells such specificities may exist; but the 

 substances or mechanisms underlying them, and the reactions which reveal 

 their existence may differ, here, in certain respects from those found in higher 

 organisms, and such differences probably exist also in regard to the genetic 

 constitution which determines these specificities. On the other hand, the results 

 achieved by these various modes of interaction of different species are very 

 similar in unicellular and the more complex organisms. As far as the indi- 

 viduality differential reactions are concerned, some very finely developed 

 mechanisms, indicative of autogenous equilibria, are found in certain in- 

 fusoria among the protozoa and also in some primitive plants; however, in 

 these unicellular organisms also, environmental conditions, in addition to the 

 genetic factors, seem to enter into the determination of these interactions to 

 a greater extent than they do in higher animals. Moreover, in unicellular 

 organisms there seems to be superimposed upon these autogenous equilibria, 

 a second type of mechanism, corresponding to the fertilization process; it 

 resembles the types of interaction which occurs between the eggs and sperma- 

 tozoa in higher organisms. In the latter, the point of equilibrium is situated 

 in the homoiogenous rather than in the autogenous zone in the spectrum of 

 relationships. Different genetic and phenotypic mechanisms underlie these 

 processes of interaction in transplantation and fertilization and these mech- 

 anisms may vary also in different organisms, as for instance, in ascidians and 

 in some plants. However, there are indications that even in fertilization 

 genetic constellations, similar to those which determine the individuality dif- 

 ferentials, may also play a part. 



