204 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



we have reasoned correctly, be much more liable to 

 death or disablement from the disease than the normal 

 inhabitants of the land ; on the other hand, should the 

 disease from any cause overpass its normal boundaries, 

 we ought to find that the inhabitants of the newly- 

 invaded territory are much more susceptible to its 

 attacks, and to death or disablement from them, than 

 the dwellers within its normal habitat. That we have 

 reasoned correctly may be proved, a 'posteriori, by a 

 great mass of evidence, but the consideration of it also 

 may conveniently be deferred while we turn our atten- 

 tion for awhile to the question of immunity. 



Two kinds of immunity, or partial immunity, from 

 the attacks of zymotic disease are distinguishable, the 

 inborn and the acquired. The first, which is trans- 

 missible to the offspring, plainly results from the sur- 

 vival of the fittest ; the second, which is not transmis- 

 sible, results indirectly from the same cause ; between 

 them they afford a beautiful example of the universal 

 truth that in higher organisms, while inborn traits are 

 transmissible, acquired traits are not transmissible, but 

 die with the individual that acquired them. As 

 regards the first kind of immunity from zymotic dis- 

 ease, the inborn inherited kind, so long as, during the 

 phylogeny, any such disease frequently causes death 

 or serious disablement, the evolution of the race 

 afflicted by it towards complete immunity will con- 

 tinue, and will do so notwithstanding that the law of 

 retrogression will cause at the same time many indi- 

 viduals to revert towards the ancestral condition of 

 non-immunity and to perish. But when the evolution 

 of the species towards complete immunity has pro- 

 ceeded so far, that an attack of the disease does not in 

 the great majority of cases result in death, or in serious 

 or long-continued disablement, the evolution will cease ; 

 an equilibrium will be established between the two 



