DIFFERENTIALS AND EVOLUTION 599 



Miller, who compared the occurrence of the ordinary human agglutinogens and 

 of M and N agglutinogens in man, the anthropoid apes, as well as in Old World 

 monkeys and New World monkeys. They could trace in this way the develop- 

 ment of certain genes corresponding to these agglutinogens within a limited 

 range of the evolutionary process. Man and anthropoid apes are most closely 

 related; accordingly, they have agglutinogens A or B, or both A and B, in 

 common. The distribution of A and B differs in different species, but it also 

 differs in different human individuals. Factors A and B have not been found 

 among Old and New World monkeys ; but among certain New World monkey 

 species and among Lemuridae a factor may occur, which is related to but 

 not identical with human B. M and N agglutinogens occur as alleles in man, 

 but in chimpanzee a combination (MN) has been found, which has not been 

 observed in man. Again there occurs in Gibbons and New World monkeys a 

 factor similar to but not identical with the human M. It has been assumed 

 that progenitors for A and B are present in common ancestors of these 

 species and that B is perhaps the older factor. 



Irwin tested by means of a series of immune sera, the number of genes 

 which were common to various species of pigeons. In comparing two of these 

 species he concluded that each of these has a number of genes which the other 

 species do not possess, and in addition there is a set of genes which both 

 species have in common. Accordingly among the Old World species of pigeons 

 there are sets of genes which are characteristic of each species, and other 

 genes are shared by these species. The same applies to the New World species 

 of pigeons, and a third set of genes is shared by Old and New World 

 pigeons. These conclusions would imply that the gene constitution of each 

 class, family, genera, species, strain, and still more, of each individual, is 

 extremely complex, consisting of numerous sets of genes which two different 

 groups share and of others which distinguish them. The nearer these species 

 or groups of species are to one another, the greater is the number of genes 

 they have in common, and the further distant they are, the smaller is the 

 number of genes which are identical. These conclusions are based on the as- 

 sumption that each agglutinogen is associated with or determined by a par- 

 ticular gene ; however, there is the possibility that each agglutinogen is deter- 

 mined by more than one gene. Furthermore, it is only one type of phenotypic 

 characters and one type of genes which have been considered in these in- 

 vestigations, namely, those which determine the agglutination of erythrocytes 

 by specific immune sera. But there are innumerable other characters which 

 are independent of the agglutinogens of erythrocytes and if the gene-deter- 

 miners of these characters were also included, the complexity in the genie 

 constitution of tissues would become still much greater. 



It seems plausible that mutations causing very fargoing changes in organs 

 and acting on earlier embryonal processes may modify some of the more 

 basic organismal differentials, which developed first in the course of evolu- 

 tion, while mutations affecting the constitution of organs, which were more 

 recently acquired and which are of a less fundamental nature, may modify 

 the individuality differentials. In general, it seems that the more similar two 



