10 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



certain gene sets rather than individual genes which represent the precursors 

 of organismal differentials. While the individuality differential is therefore 

 determined by the gene composition, it is not identical with the gene sets 

 but differs from them in a way in which other characters of the adult organism 

 differ from the gene sets. Gene hormones may mediate the effects of the genes 

 on the organismal differentials; also, other factors which form part of the 

 environment in which the organism develops may conceivably modify the 

 development of the individuality differential from its precursor substances, and 

 there are indications that adaptive processes which may take place in the inter- 

 action between host and transplant may modify these differentials, or at least 

 their manifestation. Such adaptive processes are very prominent in serial 

 transplantations of tumors. Yet there remains constant the difference between 

 the individuality differentials and the organismal differentials in general on 

 the one hand, and the differentials of specific organs and tissues on the other 

 hand; the organismal, and in particular the individuality differentials are the 

 same in all the tissues and organs within the same organism, while each organ 

 and tissue has in addition its specific differential. 



There exist, then, perhaps conditions which may complicate the direct 

 relation between organismal differentials and the gene sets which ultimately 

 determine the nature of these differentials. There are, in addition, several other 

 complicating factors. In no case is it possible to determine the organismal, and 

 in particular the individuality differentials directly, but we determine the 

 consequences of the interaction of the organismal differentials of host and 

 transplant; we observe their manifestation and this depends not only on the 

 nature of the organismal differentials but also on the amount of organismal 

 differences produced and given off by the host, and especially also by the 

 transplant, on the degree of reactivity of the host against strange differentials, 

 on the mode of attack on the part of the host, and on the ability of the graft 

 to resist these injurious conditions. A tissue that is readily injured will not 

 give off its individuality differentials for any length of time, because it will 

 be converted into a lifeless foreign body which has lost its specificity. 



Notwithstanding these difficulties it is possible to analyze the nature of the 

 organismal differentials if we carry out a number of sufficiently varied experi- 

 ments, and by these means it is also possible to follow the formation of the 

 organismal differentials in the course of phylogenetic evolution and onto- 

 genetic development, and the gradual refinement of these differentials as indi- 

 cated by the appearance and increasing significance of the individuality 

 differentials. However, this phylogenetic and ontogenetic development is not 

 represented by a straight ascending line. There are various branches given 

 off by the main line which indicate the development of mechanisms closely 

 resembling the active individuality differentials, but- which may not be 

 identical with the latter ; in such instances it may not be possible to determine 

 whether we have to deal with real organismal differentials, to which the 

 criteria we have discussed apply. In particular, it is impossible to apply this 

 term in the strict sense to unicellular free-living organisms. Thus the attempt 

 to join together the main body of a rhizopod and a pseudopod, which has 



