DIFFERENTIALS AND EVOLUTION 603 



the development of cancer. The latter represents, then, apparently a disturbance 

 of the organ and tissue equilibrium, which has ensued from the increasing 

 differentiation and specialization of tissues and organs and from the increasing 

 rigidity in the constitution of the whole organism, which took place in the 

 course of evolution. 



4) A fourth type of disease is due to the struggle between complex higher 

 organisms and various types of parasites, especially bacteria, protozoa and 

 various viruses. In this case the reaction of the host against the invader is due 

 in part to the effect of specific toxic substances, which injure certain tissues 

 and organs of the host; also, the direct destructive effect of parts of the host 

 by parasites may play a role in this disease process. But there are viruses 

 which, instead of causing a primary destruction, may induce cancerous growth 

 processes in certain hosts and in certain tissues of these hosts. Moreover, 

 parasites possess organismal differentials which differ greatly from those of 

 the host, and these differences may disequilibrate the latter and thus lead to 

 disease. However, the organism which, as the result of inadequacies in its 

 own constitution and in its interaction with the living and non-living environ- 

 ment, receives injuries and becomes diseased, is not merely a passive agent; 

 it also responds actively to the injurious factors, and these reactions may be 

 the cause of new diseases superimposed upon the primary ones. Local re- 

 actions of the host, in the form of so-called inflammatory processes, may cause 

 a sclerosis (cirrhosis) of certain organs, with serious consequences for the 

 economy of the organism as a whole. But also thrombosis, and even cancer 

 in certain respects, may be considered as reactions of the organism against 

 abnormal conditions ; furthermore, immune processes directed against strange 

 substances are, in many cases, beneficial, causing the death of the invading 

 parasite, or helping to destroy strange organismal differentials or to convert 

 the latter into the differentials of the host. However, in other cases they 

 may be destructive for the host. This occurs if reactions of a similar nature 

 to the immune processes lead to states of anaphylaxis or various kinds of 

 allergy. In these conditions, organismal differentials may also play a part and 

 there is reason for assuming that the sensitiveness to strange organismal dif- 

 ferentials becomes greater with furthergoing differentiation and specializa- 

 tion of the organismal differentials; likewise, the destruction of organs 

 becomes increasingly serious with the increasing differentiation of organs and 

 their increasing inability to restitute the lost parts with advancing evolu- 

 tion. Thus, with progressing evolution disease processes may preponderate 

 over restitutive processes, although both go hand in hand. 



The relations between the host and the various organisms which live on 

 or in the host may be that of symbiosis or parasitism. As to the role which 

 organismal and organ differentials play in these relationships, in some in- 

 stances host and symbiont or parasite may belong to the same species, and 

 this occurs in plants as well as in animals ; but as a rule they belong to very 

 distant classes and usually the host is phylogenetically a much higher or- 

 ganism than the parasite or symbiont. Also, the degree of specificity in these 

 relationships varies greatly in different cases. There may be an adaptation 



