ATTITUDE OF WORKING NATURALISTS. 241 



plains at once why contagious or epidemic diseases are 

 most fatal at their first appearance, and less so after- 

 ward : not by the dying out of a virus for, when the 

 disease reaches a new population, it is as virulent as 

 ever (as, for instance, the small-pox among the In- 

 dians) but by the selection of a race less subject to 

 attack through the destruction of those that were 

 more so, and the inheritance of the comparative immu- 

 nity by the children and the grandchildren of the sur- 

 vivors ; and how this immunity itself, causing the 

 particular disease to become rare, paves the way to a 

 return of the original fatality ; for the mass of such 

 population, both in the present and the immediately 

 preceding generation, .not having been exposed to the 

 infection, or but little exposed, has not undergone se- 

 lection, and so in time the proportion liable to attack, 

 or to fatal attack, gets to be as large as ever. The 

 greater the fatality, especially in the population under 

 marriageable age, the more favorable the condition of 

 the survivors ; and, by the law of heredity, their chil- 

 dren should share in the immunity. This explanation 

 of the cause, or of one cause, of the return of pests at 

 intervals no less applies to the diminution of the effi- 

 cacy of remedies, and of preventive means, such as 

 vaccination. When Jenner introduced vaccination, 

 the small-pox in Europe and European colonies must 

 have lost somewhat of its primitive intensity by the 

 vigorous weeding out of the more susceptible through 

 many generations. Upon the residue, vaccination was 

 almost complete protection, and, being generally prac- 

 tised, small-pox consequently became rare. Selection 

 thus ceasing to operate, a population arises which has 



