526 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



showed that immune hemolysins produced against erythrocytes of adult 

 chickens, beef, rabbits and guinea pigs are without injurious effect if they 

 are injected into the circulation of embryos or of newly-born animals of these 

 species. Similarly, very young children often prove negative to the Schick 

 and Dick tests, their blood being free from antitoxin. On the contrary, we 

 may find in young organisms a lack of resistance to certain bacteria and 

 viruses, to which adult organisms may be definitely resistant. Thus there seems 

 to be an increase in the resistance to poliomyelitis with advancing age, as 

 manifested by the fall in morbidity and the rising level in the therapeutic 

 efficiency of blood serum. In the serum of adult Rhesus monkeys a substance 

 neutralizing the poliomyelitis virus may be observed, while it is lacking in 

 immature monkeys. Such increased resistance has been attributed by some 

 investigators to a preceding latent or very mild infection with this disease. But 

 Jungeblut and Engle could show that such a change may occur even in 

 monkeys which have been kept isolated and that it is therefore probably 

 not due to a previous infection. 



Likewise, young rats are much more susceptible than are adult rats to 

 inoculation with pneumococci. The blood of almost all humans is destructive 

 for pneumococci of type II, while that of very few persons possesses the 

 power to destroy type I. The blood of an intermediate number of individuals 

 destroys type III. In the blood of young children, one to fifteen months old, 

 this ability of a mixture of serum and leucocytes to kill pneumococci is rarely 

 observed (Robertson and Sia) ; with advancing age its frequency increases, 

 but in the aged it is again rarely present. As in the case of poliomyelitis, some 

 investigators attribute also this increase in resistance with advancing age to a 

 preceding latent or mild infection with the specific organism, but recently the 

 suggestion has been made that an infection with a different and perhaps non- 

 virulent organism which has certain antigens in common with the pathogenic 

 one, may be responsible for such an effect. But this interpretation is not 

 sufficient to explain all observations, although it may apply in some instances. 

 Other investigators, Friedberger, Hirszfeld, Jungeblut and Engle, assume 

 therefore that a gradual physiological ripening, due to a biochemical change 

 in cells or tissues, is the cause of this altered mode of reaction in older age. 

 On the whole, this explanation appears more probable and it seems to hold 

 good, for instance, in the case of the blood-group differentials and in other 

 instances already mentioned. A maturation immunity develops in mice, with 

 advancing age, to the virus of vesicular stomatitis. Mice two weeks of age, 

 succumb to intramuscular injection of this virus in almost 100 per cent. With 

 increasing age, the resistance gradually increases. Mice older than six weeks 

 are completely resistant to intramuscular injection, but not to intracerebral 

 administration of this virus (Olitsky, Sabin and Cox). A barrier to the 

 further progression of the virus seems to develop at the myoneural junction. 

 In favor of the theory of a maturation resistance of tissues, it may also be 

 stated that an active immunity is very difficult to produce against certain 

 cells and toxins in very young animals. This fact has been demonstrated by 

 the experiments of Famulener, who found that young kids did not respond 



