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We may conclude, generally, that heredity and selection ought to produce an alternation of intensity in diseases. Tho alternation should be the more marked as the disease in question is more fatal, and more particularly attacks the young. Finally, the preventive or curative measures which may have sufficed in the periods of diminution must lose a portion of their efficacy in the period of aggravation, particularly as regards vaccination in its relations to smallpox. The works of Darwin beingdoi:10.1056/nejm187402190900804 fatcat:jdfmki5ucndppauuvaj2fcfeva