INTRODUCTION 5 



of one individual recognize different individuals as such, they do more than 

 that, they recognize, to speak in a metaphorical way, the degree of difference 

 between two individuals in accordance with their genetic constitution. 



It is not only the cells and tissues of one individual, however, which react 

 towards these elements of another individual in such a specific manner, but 

 there is also a substance in the bodyfluids of one individual which responds 

 towards all the cells and tissues of another individual in accordance with the 

 degree of the genetic difference between these two individuals. This again 

 indicates that there is a constituent common to all the tissues of an organism 

 which interacts with a constituent in the blood serum of another individual. 



We may designate this particular characteristic distinguishing one indi- 

 vidual from another as his individuality differential; it is common to all the 

 various tissues and organs of an individual. In the same way, there are 

 characteristics common to all members of a species, genus, order and class, 

 which may be called species-genus-order-class differentials, and these may 

 be designated in their totality, together with the individuality differentials, as 

 organismal differentials, among which the individuality differential is the 

 highest and finest one. In contrast to these, in particular to the individuality 

 differentials, are the tissues and organ differentials, which differentiate from 

 one another the different tissues and organs, such as liver, kidney, thyroid, 

 carti'age, epidermis, in the same individual. 



Theoretically it is of course conceivable that two individuals belonging to 

 the same species, other than unioval twins, possess exactly the same genetic 

 constitution and that accordingly their individuality differentials are identical ; 

 but considering the large number of genes which in all probability determine 

 this differential and considering also the possibility that mutations occur 

 spontaneously in the genetic constitution of individuals, such a state of 

 identity must be very rare indeed. Actually it has never been observed in the 

 course of our experiments which were numerous and which extended over 

 a long period of time, except possibly among brothers in a closely inbred 

 family of guinea pigs ; but even in this case the actual identity has not been 

 as yet definitely proven. However, as far as the identity of ordinary, non- 

 related individuals of the same species is concerned, the occurrence of such 

 an identity is so improbable that it has not been considered in the chapters 

 of this book, in which only the principles underlying the concept of individu- 

 ality are discussed. 



There are two principal methods by means of which the organismal 

 differentials in general can be analyzed, namely, (1) by various types of 

 transplantation, and (2) by serological methods. As to transplantation, in a 

 wider sense we may include also parabiosis, the joining together of two fully 

 formed organisms and also the uniting of parts of embryos or of blastomeres ; 

 even the transfer of a spermatozoon into the egg during the process of 

 fertilization and the joining together of parts of free-living cells, such as 

 protozoa, may be considered as types of transplantation. Transplantation 

 and serological methods are not equally well adapted to the analysis of 

 organismal differentials; each has its own sphere in which it can be applied 



