ORGANS AND TISSUES AS CRITERIA 465 



distinct from that of the latter. It may be impossible in many cases to answer 

 such a question. These difficulties arise especially if there are found chemical 

 characteristics in a certain group or tissue of an organism and if these are 

 lacking in other organs. In such a case class or species specificities, which in 

 other instances are due to the existence of organismal differentials, may be 

 due to chemical structures of a different kind in which the organismal dif- 

 ferentials are not involved. This question may arise also if we have to deal 

 with characteristics of organs and tissues which distinguish one species, or 

 one individual, from another, but in which these organ differentials do not 

 show the gradations corresponding to the degrees of phylogenetic relationship. 



While skin patterns as well as scents are characteristic of individuals and 

 may differentiate one from another, it has not been shown that these 

 structural and biochemical characteristics can be used for determining the 

 relationships between organisms belonging to the same species in the same 

 sense in which individuality differentials can be used for this purpose. This 

 fact does not exclude the possibility that as a result of close inbreeding, 

 continued through successive generations, we might approach a homogeneous 

 population, in which all component individuals would presumably have very 

 nearly the same skin patterns and scents. There are other tissue and cell 

 characters which show a certain group distribution, which is largely inde- 

 pendent of individual and species relationship. This is true, for instance, of 

 the agglutinability of the red corpuscles according to which individuals can 

 be assigned to one of the four primary blood groups ; although these char- 

 acteristics may be similar in related species such as men and certain apes. 

 There are other tissue or cell characters, such as the heterophile differentials, 

 which are distributed quite irregularly among different species, without 

 regard to relationship. Some substances show variations in constitution, which, 

 within a definite range, correspond to relationships of species ; this seems to 

 be true of the hemoglobins, and there is some reason for assuming that it is 

 true of other kinds of proteins. 



However, the larger the number of tissue and organ characteristics of 

 individuals, families and species which we use for identification, the more 

 probably will become the chance that, in their totality, their distribution will 

 correspond to relationships between these individuals, families and species. 

 Thus, if we study various organ systems in different species, a correspond- 

 ence is found, at least in a general way, between these structures and the 

 phylogenetic relationship of these species ; comparative anatomy and bio- 

 chemistry can help thus in the tracing of phylogenetic relationships, and more 

 intricate studies of the evolution of organ systems may likewise reveal 

 individual and family relationships. We may therefore conclude that various 

 kinds of tissue and organ differentials, whether structural, biochemical, or 

 functional, may serve to distinguish between individuals, and insofar as 

 these characteristics have a genetic basis, they might, in a limited way, even 

 indicate certain relationships between individuals; but they would not there- 

 fore become identical with individuality differentials. 



