404 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



apply generally is indicated by the fact that the writer, as well as Jensen, 

 observed that successive inoculation of tumors into the same animal led in 

 some cases to the growth of both tumors; and Russell, as well as Tyzzer, 

 found that there are actively growing tumor grafts which apparently do not 

 produce immunity in the host. Russell distinguished, therefore, tumors which 

 as a result of their growth, conferred concomitant immunity, and others which 

 did not confer such an immunity. In addition, there were tumors which 

 showed an intermediate behavior. However, there is reason for assuming 

 that all growing homoiogenous tumors are able to induce active immunity 

 against secondarily transplanted homoiogenous tumors, but that the degree of 

 this immunity varies in different cases and that the presence of such an im- 

 munity may not always become manifest. If a tumor originates in a strain of 

 animals in which, as the result of long-continued, very close inbreeding, the 

 individuality differentials in the various animals have become very similar, 

 then the individuality differentials of tumors which originate in such a strain 

 are about the same as those of the animals constituting this strain, and such 

 tumors after transplantation into members of this homozygous strain behave, 

 therefore, about like autogenous tumors, which have no antigenic power. 



After transplantation into a strange, closely inbred strain, the large ma- 

 jority of tumors do not show continued growth ; they may grow for a short 

 time and then retrogress and disappear, or may show a slightly longer initial 

 growth. Similarly after transplantation into only partly inbred strains, not 

 yet approaching homozygosity, the majority of primary (spontaneous) tumors 

 do not take. But as we have discussed already in the preceding chapters, dif- 

 ferent tumors differ very much in this respect. There are some tumors which 

 can be transplanted into almost all homoiogenous animals and others which 

 may be transplanted in various proportions into such animals. Furthermore, 

 there exist differences between different strains of animals serving as hosts ; a 

 certain tumor which originated in an American mouse may be transplanted 

 into a considerable number of American mice, but not at all or only into a very 

 small number of some European strains. These differences in transplantability 

 depend partly on the relations and the degrees of mutual strangeness between 

 the individuality differentials of hosts and transplants; but there enter also 

 other factors, such as the growth momentum of the tumors, variations in their 

 sensitiveness and power of resistance to injurious substances, and lastly, in 

 their ability to adapt themselves to new hosts bearing individuality differen- 

 tials of different degrees of strangeness. Moreover, different strains of hosts 

 and different individual animals may not have the same ability to react against 

 and to injure a transplant carrying a strange individuality differential. While 

 the significance of these factors has not yet been analyzed sufficiently in the 

 recorded series of transplantations, and while it is not yet possible to determine 

 in most cases how much importance is to be attributed to one or the other of 

 these factors, there is enough evidence at hand to warrant the conclusion that 

 they play a role under various conditions. 



To return now to the discussion of concomitant immunity. We have seen 

 that the prerequisite for the development of this type of immunity is a differ- 



