346 



SCIENCE. 



[N. S. Vol. I. No. 13. 



that the tei-miual lobation is most clearlj^ 

 shown. A comparison of this figure with 

 the numerous specimens of LiriodendrojJS'k 

 simplex of Newberry leaves no doubt what- 

 ever that the Portuguese plant is at least a 

 congener of the American plant, and it is 

 just possible that it may belong to the same 

 species. As this form has been three times 

 published* it is a little surprising that 

 Saporta did not think to compare it with 

 the Portuguese plant. There are differences 

 in the finer nervation, but this is also per- 

 ceptible between his two drawings of the 

 same specimen ; these also differ in different 

 specimens of the American plant, and one 

 or two other species remain to be published. 

 When all the material is illustrated most 

 of these differences will disappear. If any 

 remain it can be ascribed to difference of 

 age and geographical position. 



Lester F. Ward. 

 Washington. 



EXPLANATION OF ACQUIS ED IMMUNITY 

 FROM INFECTIOUS DISEASES.i 



It has long been known that, in a con- 

 siderable number of infectious diseases, a 

 single attack, however mild, affords protec- 

 tion against subsequent attacks of the same 

 disease ; that in some cases this protection 

 appears to be permanent, lasting during the 

 life of the individual ; that in others it is 

 more or less temporary, as shown by the 

 occurrence of a subsequent attack. 



The protection afforded by a single attack 

 not only differs in different diseases, but in 

 the same disease varies greatly in different 

 individuals. Thus certain individuals have 

 been known to suffer several attacks of 

 small-pox or of scarlet fever, although, as a 



*BuU. Ton-. Bot. Club, Vol. XIV., New York, 

 Jan. 1887, p. 6, pi. Ixii, figs. 2, 3, 4 ; Am. Journ. 

 Sci., Vol. XXXIX., New Haven, February, 1890, p. 

 98, pi. ii., figs. 6, 7 ; Trans. N. Y. Acad, Sol., Vol. 

 XI., 1892, p. 102, pi. it., figs. 2-7, 9. 



t Abstract of a paper read before the Biological 

 Society of Washington, March 9, 1895. 



rule, a single attack is i^rotective. Excep- 

 tional susceptibility or insusceptibility may 

 be not only an individual but a familj' char- 

 acteristic, or it may belong to a particular . 

 race. 



In those diseases in which second attacks 

 are not infi-equent, as, for example, in pneu- 

 monia, in influenza or in Asiatic cholera, 

 it is difficult to judge from clinical experi- 

 ence whether a first attack exerts any pro- 

 tective influence. But from experiments 

 upon the lower animals, we are led to be- 

 lieve that a certain degree of immunity, 

 lasting for a longer or shorter time, is af- 

 forded by an attack of pneumonia or of 

 cholera, and probably of all infections due 

 to bacterial parasites. In the malarial 

 fevers, which are due to a parasite of a 

 different class, one attack affords no pro- 

 tection, but rather predisposes to a subse- 

 quent attack. 



In those diseases in which a single at- 

 tack is generally recognized as being pro- 

 tective, exceptional cases occur in which 

 subsequent attacks are developed as a re- 

 sult of unusual susceptibility or exposure 

 under circumstances especially favorable to 

 infection. Maiselis has recently (1894) 

 gone through the literature accessible to 

 him for the purpose of determining the fre- 

 quency with which second attacks occur in 

 the various diseases below mentioned. The 

 result is as follows: 



Second Tliird Fourth 



Attacks. Attacks. Attacks. Total. 



Small-pox . . 505 9 514 



Scarlet fever .29 4 33 



Measles ... 36 1 37 



Typhoid fever . 202 5 1 208 



Cholera ... 29 3 2 34 



Eecent researches indicate that the prin- 

 cipal factor in the production of acquired 

 immunity is the presence, in the blood of 

 the immune animal, of some substance ca- 

 pable of neutralizing the toxic products of 

 the particular pathogenic microorganism 



