DIFFERENTIALS AND EVOLUTION 605 



the large majority of the tissues. This organ-specificity suggests the possi- 

 bility that a definite species distribution of the parasites may not be due to 

 the specificity of the organismal differentials of the hosts, as for instance in 

 certain cases in which the parasites live and propagate only in a single species 

 or in a very few species, but is due rather to peculiarities which organs in 

 different species possess. The term organismal differentials would therefore 

 be used here in a wider sense. 



While there exists in the relations between host and transplant frequently 

 an organismal- as well as an organ-specificity, in some instances the organ- 

 specificity, in others the organismal-specificity may predominate. But these 

 specificities are not always rigidly fixed; they may be modifiable through 

 serial passages of the parasite in a host species other than the one to which 

 it has been originally adapted. Gradually a change may take place in the 

 relative virulence of the parasite for various host species; this change in 

 species-specificity can be obtained also by means of many passages through 

 the chick chorio-allantoic membrane or the chick embryo, and not only the 

 species-specificity may be diminished or altered by this procedure, but also 

 the organ-specificity may be markedly decreased. These effects can be studied 

 very well in various viruses. There is therefore noticeable, here, a great 

 similarity between the behavior of certain viruses and of tumor transplants ; 

 the latter can become adapted to new hosts through many consecutive passages 

 in different hosts. Moreover, in heterogenous tumor transplants, a good 

 growth has been observed in the chorio-allantoic membrane of the chick; the 

 same is true of the growth of heterogenous normal tissues. In both viruses 

 and tumors an adaptation to a new host occurs in the course of long-continued 

 transplantations and the chick embryo and chorio-allantoic membrane seem 

 to lack the power to injure viruses or tissues and tumors possessing heterog- 

 enous organismal differentials. It remains still to be determined how far these 

 similarities in the behavior of microorganisms and viruses, on the one hand, 

 and mammalian as well as avian tissues and tumors, on the other, depend on 

 similar mechanisms. 



The evolution of the organ systems and that of the organismal differentials 

 has led to the formation of very complex, rigidly integrated organisms, in 

 which the various organs and tissues are highly specialized. These processes 

 have also resulted in an increased differentiation between organisms belonging 

 to the same species and therefore in an increased individualization. In this 

 individualization, different organ systems have had an unequal part. The 

 generative system is less important for the individual than for the continuation 

 of the species, and the so-called vegetative organ systems are essential for 

 the life and function of the individual, but are not individualized to the 

 highest degree. It is the nervous system in its interaction with the other organ 

 systems, and especially with the endocrine organs, whose development in the 

 course of evolution has made possible the greatest individualization. Increasing 

 differentiations in the nervous system, in its cooperation with the hormone 

 system, have made possible the coordination and correlation of the functions 

 of the various organs and tissues belonging to the same organ system and 



