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A familiar illustration of the restrictions on his experience of it is to be found in the rule which compels the calling of “experts” when there is a question as to any point of science or art. “The words science or art,” says Mr. Fitzjames Stephen, “include all subjects on which a course of special study or experience is necessary to the formation of an opinion,” and the opinion of such an expert is a “relevant fact.” So that Dr. Taylor’s “distinguished legal friend,” if a good lawyer, would not, in spite of his proficiency in circumstantial evidence, undertake to dispute with Professor Huxley about the relation of the anchitherium, hipparion, and horse; and if Dr. Taylor offered himself for examination on such a point he would be laughed out of court. In none of our courts is the presentation allowed of all the circumstances which strengthen or weaken a probability.

A lawyer, therefore, though he might not be as ill fitted for a scientific discussion as a minister, is, as such, hardly more of an authority on the force and limits of that portion of scientific proof which is drawn from simple observation. Dr. Taylor’s consulting one as a final authority as to the very nature of the argument on which he was himself about to sit in judgment is at the outset a suspicious incident. The definition of circumstantial evidence which he got from his legal friend was this:

“The process of proof by circumstantial evidence consists in reasoning from such facts as are known or proved, and thence establishing such as are conjectured to exist. The process is fatally vicious, first, if any material circumstance from which we seek to deduce the conclusion depends itself on conjecture; and, second, if the known facts are not such as to exclude to a reasonable degree of certainty every other hypothesis.”

“Now, tried by these two tests,” says Dr. Taylor, “the professor’s argument was a failure.” Taking this definition as it stands, however, we think it will not be difficult to show that Dr. Taylor is not competent to apply the tests, or to say whether the professor’s argument is a failure or not.

It is hardly necessary to say that all the evidence in our possession or attainable, with regard to the history of the earth and of animal and vegetable life on its surface, is circumstantial evidence. The sciences of geology, palaeontology, and, to a certain extent, biology are sciences of observation, and but few of their conclusions can be reached or tested by experimentation. They are the result of a collection of facts, observed in various places, at various times, and by various persons, and variously related to other facts; and the collection of these facts, and the arrangement of them, and the formation of a judgment as to their value both positive and relative, form the greater portion of the work of a scientific man in these fields. Professor Huxley’s argument, which Dr. Taylor disposes of so summarily, consists of a series of inferences from facts so collected and arranged. They are the things “known or proved,” on which, as his legal friend truly says, the reasoning in the process of proof by circumstantial evidence must rest.

Now, Dr. Taylor, by his own confession, is no authority in either geology, biology, or palaeontology. He has neither collected, observed, nor experimented in these fields. He does not know how many facts have been discovered in them, or what bearing they have on other facts in other fields. Therefore, he is entirely unable to say whether Huxley is arguing from things “known or proved” or not. Moreover, he does not, for similar reasons, know whether Huxley’s process has been “fatally vitiated” by the dependence of any “material circumstance” on conjecture, or by the insufficiency of the “known facts” to exclude every other hypothesis; for, first, he does not know what is in geological, biological, or palaeontological induction a “material circumstance”–nor does any man know except by prolonged study and observation–and, second, he does not know whether “the known or proved facts” are sufficient to exclude every other hypothesis, because he neither knows what facts are known nor what is the probative force of such as are known. We can, however, make Dr. Taylor’s position still clearer by a homely illustration. A wild Indian will, owing to prolonged observation and great acuteness of the senses, tell by a simple inspection of grass or leaf-covered ground, on which a scholar will perceive nothing unusual whatever, that a man has recently passed over it. He will tell whether he was walking or running, whether he carried a burden, whether he was young or old, and how long ago and what hour of the day he went by. He reaches all his conclusions by circumstantial evidence of precisely the same character as that used by the geologist, though he knows nothing about the formal logic or the process of induction. Now, what Dr. Taylor would have us believe is that he can come out of his study and pass judgment on the Indian’s reasoning without being able to see one of the “known facts” on which the reasoning rests, or appreciate in any degree which of them is material to the conclusion and which is not, or even to conjecture whether, taken together, they exclude the hypothesis that it was not a man but a cow or a dog which passed over the ground, and not to-day but yesterday that the marks were made.