THE PRESENT EVOLUTION OF MAN — PHYSICAL 205 



wan ill Of forces of evolution and retrocession, and the 

 species in respect to inborn immunity will remain 

 stationary. For which reason it is that complete inborn 

 immunity is not attained against such very common 

 diseases as measles and chicken-pox, which almost 

 every individual in Europe suffers from, but which are 

 only fatal to a very few — to those, speaking generally, 

 who have reverted, by the lapsing of inborn variations, 

 to the ancestral condition of non-immunity. 



As regards acquired immunity : it is a well-known 

 fact, that duration of the attack in many zymotic dis- 

 eases is limited in the individual ; that is, if the indi- 

 vidual attacked does not die, he recovers within a cer- 

 tain pretty definite period of time, which is short in some 

 diseases, — e. g. measles, — longer in others, — e.g. typhoid, 

 — and longer still in a third class — c. g. syphilis ; but cer- 

 tain other diseases — e. g. tuberculosis and leprosy — may 

 persist in the individual for an indefinite time. It is 

 also well known that in all diseases which have a 

 limited duration in the individual, one attack confers 

 for a shorter or longer time complete or partial 

 immunity against subsequent attacks. This is the 

 second kind of immunity, the kind which is not 

 transmissible, but is acquired anew by each individual ; 

 for example, one attack of small-pox is usually followed 

 by complete or partial immunity from subsequent 

 attacks of that disease, the individual attacked, in the 

 vast majority of cases, not taking the disease again, or 

 taking it only in a mild form a considerable time after 

 the first attack. Never, or so very rarely that it is 

 regarded as a medical curiosity, is one attack closely 

 followed by another. 



Now it is plain, if such a disease as small-pox or 

 measles persisted for an indefinite time in an individual, 

 or if an individual were subjected to repeated attacks 

 of the disease, that either condition would be equivalent 



