9HMKJM - The Complete Work of Charles Darwin Online

ATTITUDE OF WORKING NATURALISTS. 241
plains at once why contagious or epidemic diseases are
most fatal at their first appearance, and less so afterward
: 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 practised,
small-pox consequently became rare. Selection
thus ceasing to operate, a population arises which has