ADAPTATION OF TISSUES AND THEIR PRODUCTS 475 



It seems that specific adaptations of the kind mentioned here may underlie 

 also some types of parasitism, the parasite becoming adapted to certain sub- 

 stances of the host which carry the species differential of the host, or at least 

 differentiate one type of host from other types of hosts. Thus, according to J. 

 H. Welsh, the freshwater mussel, Anodonta cataracta Say, is infested with 

 parasitic water mites (Unionicola Ypsilophorus), which live between the gills 

 of their host. In the free-living state these mites are positively heliotropic, but 

 if to the water in which a positive heliotropic reaction would otherwise take 

 place, an extract of the gills of the host or water from the mantle cavity of 

 the host is added, they become negatively heliotropic, thus assuming the 

 characteristic behavior they show in their parasitic life. It is interesting to 

 note that in the case of Unionicola, which parasitizes on Anodonta, only 

 material from this particular host will bring about such a change in be- 

 havior, whereas corresponding substances from other species, such as Ellipho 

 or Lampsilis, have no effect on the parasite. In this instance we have to deal 

 evidently with a specific adaptation between host and parasite, which depends 

 upon the interaction of certain specific substances. However, whether the 

 substances, which play the decisive role in these and certain other cases, 

 actually carry the organismal differentials, or are merely derivatives of or 

 otherwise related to these differentials, cannot be decided without further tests. 

 But it could be made probable that the substances concerned in these reactions 

 are at least nearly related to the organismal differentials of the parasite and 

 host if, after immunization with these substances, the antibodies produced 

 were found to react not only with the material which served as antigen, but 

 also with other substances obtained from the same host species, but not from 

 distant species; or, if it could be shown that there is a graded response of 

 the parasite to analogous substances from different species, the response being 

 the stronger the more nearly related the species from which the test substance 

 is obtained, to the host species of that particular parasite. 



In the examples which we have cited, we have to deal primarily with pre- 

 formed relations between two substances, or between a substance and a tissue, 

 the reaction depending upon the genetic relationship between the organismal 

 differentials of the organisms concerned, although primarily, organs and tis- 

 sues and organ-specific substances are involved in the majority of these reac- 

 tions rather than purely individual and species-specific substances. The great 

 structural and functional specificity which is characteristic especially of the 

 higher, more differentiated organisms, depends largely upon this interaction 

 of organismal differentials or of substances derived from them, or also of 

 substances originating in organs which are specific for a species in a similar 

 manner in which the organismal differentials are specific, but which differ 

 otherwise from the latter. In addition, we have cited some instances in which, 

 by means of artificial immunization, the same specific relations between differ- 

 ent organisms, or between the substances derived from them, can be demon- 

 strated in case one of the substances involved served as antigen. 



These data may then be interpreted as indicating the presence of autoge- 

 nous species or class equilibria, in which the various organs and substances 



