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 newlyinvaded 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, à posteriori, by a great mass of evidence, but the consideration of it also may conveniently be deferred while we turn our attention 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 transmissible to the offspring, plainly results from the survival of the fittest; the second, which is not transmissible, 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 disease, 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 continue, and will do so notwithstanding that the law of retrogression will cause at the same time many individuals to revert towards the ancestral condition of non-immunity and to perish. But when the evolution of the species towards complete immunity has proceeded 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
