CHAPTER VII
EVIDENCE
After you have marshaled the facts you wish to prove, you must consider the classes of evidence by which you wish to prove them.
In the first place there is the direct statement of the facts by witnesses, or the opinions of those witnesses who are qualified to speak as experts. Then there is circumstantial evidence, which consists of inferences fairly drawn from facts; in other words reasoning about facts. Thus if you wish to prove that cutting the forest off the northern part of the lower peninsula of Michigan has lessened the rainfall in that State, you could present three classes of testimony. You could bring forward an old resident who had known Michigan when it was wooded and when it was stripped of its timber; if from his own personal observation he could testify that as the timber was cut off the rainfall had diminished, that would be direct evidence of that fact. If you presented the statements of a scientist who would testify that when lands were stripped of their trees, there was less rainfall, his opinions would be entitled to consideration as proof of the facts just in the proportion that his observation and experience had been extended and at the same time exact.
Circumstantial Evidence.—If the cutting of the forests in Minnesota near the head waters of the Mississippi was under discussion, you might present the testimony of a man who lived on the banks of that river in Iowa, who had never seen Minnesota, but who could testify that the volume of water in the river had decreased from year to year in certain proportion. His testimony would be valuable as tending to establish your position, if you could also show that the cutting of the timber in Minnesota had proceeded in the same ratio as the decrease in the river’s volume. It would then be a fair inference that the two facts were so connected that one tended to prove the other.
The line, however, between direct and circumstantial evidence is very faint—one imperceptibly glides into the other. Indeed some experts insist that there is no such thing as direct evidence. They urge that all proof consists of deductions which may be so closely related to the fact to be established that a mere statement conclusively shows the causal relation. On the other hand this relation can frequently be shown only by the most delicate fitting together of all the links of circumstance. For example, suppose the testimony of A is that he saw B point a gun at C, that the gun was fired, that immediately C fell dead. His testimony would be, therefore, that B shot C and you, if you were on the jury, would perhaps feel justified in finding B guilty of murder—there would seem to be no doubt about it. But after all, his testimony is not direct evidence—even as simple and apparently as conclusive a statement as his might be explained away. Suppose, for example, an autopsy showed that C had heart disease and fell dead from that cause, and that the bullet penetrated C’s arm only. What to the observer would seem like irrefutable proof would be dissipated in a moment; it would be shown to be an unwarrantable deduction from certain circumstances.
Remember, however, that the value of evidence is in this ratio: that direct evidence of a witness who knows of his own knowledge and observation in the particular case is more to be trusted than expert evidence based on general observation, and that both are worth more than that evidence which is the result of deduction and inference.
Qualifications of Witnesses.—In weighing the value of the testimony offered to prove your facts—testimony is those statements of the witness which make up his evidence—you must ask him certain questions. If he were a witness in a law suit, the lawyers would bring out the points you must determine by four questions. Following their example, you will first ask “Is he honest or prejudiced?” In the conservation question considered, you would ask whether he was the agent of a lumber company whose statements would naturally be influenced by his selfish interest, or a homesteader who, since his only use for land was for farming purposes, would likely be as prejudiced the other way, or was he a banker or merchant who served both classes equally, whose interest lay equally with each party to the controversy?
You will next ask if his testimony is consistent with known facts. If he testified that the absence of trees had nothing to do with rainfall and had cited as proof of that alleged fact that there was a heavy rainfall in Sahara where there were no trees, you could at once dismiss him as an impossible witness because his statements as to Sahara were inconsistent with the known facts.
You should then inquire under what circumstances were the statements made: were they forced from him, were his relations such that he was a voluntary and willing witness endeavoring to assist investigation and find out the facts? If so, his testimony is probably valuable and worthy of credence.