I shall place before you three kinds of evidence entirely based upon what is known of the forms of animal life which are contained in the series of stratified rocks. I shall endeavor to show you that there is one kind of evidence which is neutral, which neither helps evolution nor is inconsistent with it. I shall then bring forward a second kind of evidence which indicates a strong probability in favor of evolution, but does not prove it; and, lastly, I shall adduce a third kind of evidence which, being as complete as any evidence which we can hope to obtain upon such a subject, and being wholly and strikingly in favor of evolution, may fairly be called demonstrative evidence of its occurrence.
GEORGE M. STERNBERG, M.D., L.L.D, SURGEON-GENERAL U.S. ARMY
This article is a scientific demonstration of a new fact. It shows clearly the processes of scientific reasoning based on the methods known to Logic as the Methods of Agreement and Difference. The theory that the germs of the disease are carried by mosquitoes seems first to have suggested itself to Dr. Sternberg and to Dr. Finlay through noticing a similarity of phenomena in many cases under different conditions. Yet, however plausible, the theory, neither of them could declare that he had discovered the fact until the experiments carried on under rigorous precautions had been tried. By these experiments all other causes were ruled out of consideration.
The discoveries which have been made in the past twenty-five years with reference to the etiology of infectious diseases constitute the greatest achievement of scientific medicine and afford a substantial basis for the application of intelligent measures of prophylaxis. We know the specific cause ("germ”) of typhoid fever, of pulmonary consumption, of cholera, of diphtheria, of erysipelas, of croupous pneumonia, of the malarial fevers, and of various other infectious diseases of man and of the domestic animals, but, up to the present time, all efforts to discover the germ of yellow fever have been without success. The present writer, as a member of the Havana Yellow Fever Commission, in 1879, made the first systematic attempt to solve the unsettled questions relating to yellow fever etiology by modern methods of research.
Naturally the first and most important question to engage my attention was that relating to the specific infectious agent, or “germ,” which there was every reason to believe must be found in the bodies of infected individuals. Was this germ present in the blood, as in the case of relapsing fever; or was it to be found in the organs and tissues which upon post-mortem examination give evidence of pathological changes, as in typhoid fever, pneumonia, and diphtheria; or was it to be found in the alimentary canal, as in cholera and dysentery?