604 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



of parasite or symbiont to one particular kind of host, or an adaptation to 

 a number of phylogenetically related hosts or to very diverse hosts, or the 

 two latter adaptations may exist at the same time. Parasites of animal or 

 plant origin as wellas viruses may live and propagate in or on very distant 

 organisms ; there does not need to exist an exact relationship between the 

 organismal differentials of host and parasite corresponding to phylogenetic 

 evolution. 



However, under certain conditions organismal differentials may play a 

 certain role in determining the invasion of the hosts by the strange organisms. 

 The relationship between the organismal differentials of host and parasite 

 or symbiont may resemble that of certain organ and tissue differentials, or 

 the distribution of blood-group or Forssman differentials in various species. 

 But whatever the significance of organismal differentials in these relation- 

 ships may be, parasites and symbionts are usually adapted to definite host 

 species and often also to definite organs or tissues within a certain species. 

 There exists therefore a marked specificity in the relations between host and 

 invader. This specificity may be so great that it is possible to distinguish 

 between different strains of hosts by determining the kind of parasites or 

 symbionts which live on or in them, and conversely, to distinguish between 

 nearly related parasites or symbionts by determining the host on which they 

 are found. The mutual relationship between the organismal differentials of 

 host and parasite may be one of the factors which determine the interaction 

 between these two organisms; this interaction does not depend however on 

 the organismal differentials of either host or parasite alone. In this respect 

 the relationship between host and parasite resembles that between host and 

 transplant, which depends on the organismal differentials of both host and 

 graft. 



There are indications that the specific adaptation between host and parasite 

 or symbiont may be due partly, at least in some cases, to the presence of certain 

 substances in these two organisms which are specifically adapted to each 

 other. Furthermore, related parasites may contain related antigens, which 

 may call forth the production of antibodies showing cross-reactions with the 

 antigens of these parasites. A very instructive observation pointing to the 

 presence of specifically adapted substances in host and parasite, which make 

 possible this condition of parasitism, has been made by Welsh, who found 

 that various species of mites which live between the gills of Anodonta and 

 other mussels are positively heliotropic when they are removed from their 

 normal habitat. Addition of extract of the gills or of fluid from the mantle 

 cavity of the species on which they live makes this heliotropic reaction nega- 

 tive, and it is only extract or fluid from the species to which they are adapted 

 which has this effect and not substances obtained from other species of 

 mussels. 



As mentioned, there is noticeable in many cases also a distinct organ- or 

 tissue-specific adaptation between host and parasite or symbiont. However, 

 in this respect also, great differences exist in different parasites; some are 

 adapted to a single organ or tissue, others can live and multiply in several or in 



