DIFFERENTIALS AND EVOLUTION 597 



two individuals belonging to the same inbred family or strain can be gradu- 

 ally increased, it is very difficult to achieve complete identity as it exists be- 

 tween different parts of the same organism. This identity of the individuality 

 differentials of different tissues and organs in the same organism can be 

 demonstrated, notwithstanding the existence of great differences between 

 different tissues and organs. 



It is primarily the difference in individuality differentials of the individuals 

 belonging to the same species which causes the reactions of the host against 

 the transplant, the local as well as the distant reactions, and which also may 

 cause immune reactions in an animal after introduction of tissue or its con- 

 stituent substances or of bodyfluid belonging to a not closely related in- 

 dividual ; parts of the same individual do not elicit either a contact or a distance 

 reaction after transplantation ; nor do autogenous substances elicit an immune 

 reaction, except perhaps parts of the body which, in certain respects, are 

 separated from and strange to the other parts of the organism, and in par- 

 ticular products of degeneration, which may differ in constitution from the 

 living parts. Organ differentials and artificial partial antigens, as a rule, 

 function as full antigens only in combination with strange individuality or 

 preferably with strange species and order differentials. It is especially the 

 strange organismal differentials which interfere with the integrated function 

 of the host organism into which they are introduced and which make it possible 

 for the host to react also against specific structures other than organismal 

 differentials. 



As to the progressive evolution in structure, chemical constitution and 

 function of tissues and organs, and in the constitution of the whole organism, 

 it is assumed by geneticists, and also by some other students of evolution, that 

 this is caused by mutations, alterations in chromosomes and genes, in associa- 

 tion with processes of segregation and selection. If the conclusion is accepted 

 that mutations are the primary means through which organisms change in 

 the course of evolution, then it would be further necessary to assume that 

 changes in organs, caused by mutations, will affect also the organismal differ- 

 entials in the course of time. As far as the individuality differentials are con- 

 cerned, there is reason for believing that these depend, as already stated, 

 upon very many genes and it may therefore be assumed that a change in a 

 single gene, which might be sufficient to induce a modification in the struc- 

 ture and function of a certain organ or tissue, would not alter the individuality 

 differential noticeably, or only to a very slight degree ; but repeated mutations 

 might produce a more marked effect on the individuality differential. Such 

 modifications in the genetic constitution would in many instances affect only 

 superficial mechanisms, which do not control vital processes in the adult, and 

 they would affect, first, the late stages in embryonal development. Secondarily, 

 however, such changes might influence also other mechanisms in the organism 

 and thus alter a variety of characters. As to mutations which result in slightly 

 further-going changes, such as those which have led to the transformation of 

 gray Norway rats to "Mutant Albino" or to "Curly Coat," which were ob- 

 served by H. D. King, even these do not seem to change the organismal dif- 



