DIFFERENTIALS AND EVOLUTION 595 



antigens, with the exceptions already mentioned, as well as in the case of the 

 melanin pigments. 



There are, then, morphological and metabolic characteristics of tissues and 

 organs which, to a high degree, seem to be correlated with the gradation and 

 relationship of the organismal differentials; but these characteristics are 

 limited to certain organs and tissues and they are not common to all the 

 tissues, organs and organ functions of an organism. There are other structural 

 and metabolic characteristics of tissues and organs which are only partly 

 correlated with the organismal differentials and with the phylogenetic develop- 

 ment, and still others are only slightly or not at all correlated. But, it is only 

 in the larger groups, such as classes, orders, genera, that the morphological 

 and biochemical evolution of certain organ and tissue systems can be correlated 

 with the course of the phylogenetic evolution and with the evolution of 

 organismal differentials. If we study individual organisms, the distribution of 

 organ and tissue characteristics is independent of the individuality differen- 

 tials. In brothers and sisters there are structural, biochemical differences in cer- 

 tain organs and tissues, as well as psychical differences, which do not parallel the 

 relations of their individuality differentials ; this is true also of the distribution 

 of the original blood groups. There is reason for the conclusion that the organ- 

 ismal differentials have a closer and much more direct correspondence to 

 phylogenetic relationship than the organ and tissue differentials. 



There are, in addition, certain specific functional or structural relationships 

 between some cells and tissues, which very closely correspond to the relation- 

 ships between the organisms from which these cells and tissues are derived, 

 but which are not identical with the primary, typical organismal differentials. 

 Thus in some instances there exist between germ cells, spermatozoa and ova, 

 or between germ cells and certain somatic tissues, specific relations which 

 make possible the distinction between autogenous and homoiogenous relation- 

 ship. Likewise among infusoria there are mechanisms which enable these 

 organisms to distinguish the autogenous, homoiogenous or heterogenous 

 nature of parts of these organisms. We have in this, as well as in other similar 

 cases, to deal with processes which have developed not in the direct line of 

 phylogenetic evolution but in side branches and which are peculiar to them ; 

 in particular, in unicellular organisms, it is not certain what role is played 

 by genetic factors and what by cytoplasmic modifications in such mechanisms. 

 In all probability many other mechanisms of a similar nature exist, which 

 make the interactions between different organisms or between parts of them 

 specific for species, varieties or individuals. In different cases the mode of 

 manifestation of these specificities may vary, and likewise the mechanism by 

 means of which the mutual adaption of cells and tissues is produced may vary. 



An organism consists, then, ultimately of systems of graded substances, 

 some of which possess a very great organismal-specificity while others are 

 almost exclusively organ-specific, and still others show combinations of organ- 

 ismal and organ differentials, varying quantitatively in different instances. In 

 addition, there occur substances which are specific for a certain species or 



