564 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



It is well known that some bacteria are quite harmless parasites for certain 

 mammalian species, while others are very injurious, apparently without 

 regard to the phylogenetic relationship between microorganisms and hosts. 

 Special mechanisms apply here which are contingent, in part at least, on the 

 relations of these microorganisms and their toxins to specific organs. More- 

 over, mechanisms which differ in the case of different toxins may make a 

 certain substance toxic for a given organ in one species and innocuous for 

 the corresponding organ in another species. Hence it seems that the injurious 

 effect of tetanus toxin for some species depends upon the power of the brain 

 substance in this species to bind this toxin. In a more resistant species the 

 brain may have a diminished affinity for this toxin. Furthermore, the degree 

 of injuriousness of certain microorganisms, and of the substances given off 

 by them, depends upon primarily, preformed mechanisms as well as upon 

 secondary, acquired immune mechanisms, which latter may become effective 

 as the result of a primary interaction between host and parasite, leading to 

 injury in the host. The effects may also vary in very young and in adult or 

 old host organisms. 



The importance of both species and organ in determining the activity 

 of microorganisms is especially clear in the case of certain fungi or bacteria, 

 which function as symbionts in some species of insects. There is, here, an 

 adaptation not only to a particular species of insects, but also to a particular 

 receptive organ, a mycetoma, which has been formed from the fat tissue 

 surrounding the digestive tract in this species and which is especially suitable 

 for the life of the symbionts. These symbionts are found only in this organ 

 and in one other location in the hosts. The mycetoma is not produced in 

 response to the presence of the symbionts, as might have been assumed, but 

 it develops even when they are lacking. If now the mycetoma is transplanted 

 from the larvae of a species, such as Periplaneta or Psylla, to which these 

 microorganisms are adapted, to the larvae of another species, for example, 

 Tenebrio, to which they are not adapted, the transplant may remain alive 

 throughout the life of these larvae; but such transplanted symbionts manifest 

 no activity in their new hosts, in contrast to the activity in the old host to 

 which they had become adapted. 



The relations of microorganisms and their toxins to hosts are, then, in a 

 general way comparable to those of poisonous reptiles, amphibia and other 

 poisonous animals and their toxins to various species. These relations do not 

 depend directly upon the organismal differentials of host and symbiont, 

 parasite or toxic substance, although in certain instances phylogenetic rela- 

 tionships may play a limited role. The factors which determine the interaction 

 between hosts and symbionts, parasites and toxins, are in some respects 

 comparable to the Forssman differentials, which occur without regard for 

 phylogenetic relationship. The relations between toxins and organism are 

 essentially of an organ-specific character ; but there may perhaps to a limited 

 extent also organismal differentials be involved; the toxins show specific 

 adaptations to the species in which they are produced, and there is a notable 

 correspondence in the relations of toxins and of parasites or symbionts in gen- 

 eral to various species acting as receptors for the toxins, or as hosts for the 

 parasites or symbionts. 



