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this state has been evolved by a natural process from an antecedent state, and that from another, and so on; and, on this hypothesis, the attempt to assign any limit to the series of past changes is, usually, given up.

The first two hypotheses here are stated, of course, in order that they may be shown to be untenable. Once they are shown to be un enable, there will be a strong presump ion in favor of the third.

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108. The hypothesis. -The one decisive test of a good hypothesis is its complete accordance with facts. If it is not in agreement with known facts, or is inadequate to the explanation of all the facts it is required to explain, it must be discarded, and some more probable hypothesis adopted. That this test may be applied, however, the hypothesis must be of such a nature as to admit of deductions being made from it. "An hypothesis from which nothing can be deduced, . . . is of no value whatever. It always remains at the stage of mere possibility, and without any real connection with fact. It is a mere guess which has no significance whatever, for it is entirely incapable either of proof or of disproof."1

Such a useless hypothesis is that, for example, stated first in the following passage:

The adaptation of the external coloring of animals to their condition of life has long been recognized, and has been imputed either to an originally created specific peculiarity, or to the direct action of climate, soil, or food. Where the former explanation has been accepted, it has completely checked 1 J. E. Creighton, Introductory Logic, p. 242.

inquiry, since we could never get any further than the fact of the adaptation. There was nothing more to be known about the matter. The second explanation was soon found to be quite inadequate to deal with all the varied phenomena, and to be contradicted by many well-known facts. For example, wild rabbits are always of gray or brown tints well suited for concealment among grass and fern. But when these rabbits are domesticated, without any change of climate or food, hey vary nto white or black, and these varieties may be multiplied to any extent, forming white or black races. Exactly the same thing has occurred with pigeons; and in the case of rats and mice, the white variety has not been shown to be at all dependent on alteration of climate, food, or other external conditions. In many cases, the wings of an insect not only assume the exact tint of the bark or leaf it is accustomed to rest on, but the form and veining of the leaf or the exact rugosity of the bark is imitated; and these detailed modifications cannot be reasonably imputed to climate or food, since in many cases the species does not feed on the substance it resembles, and when it does, no reasonable connection can be shown to exist between the supposed cause and the effect produced.1

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109. Verification. The verification or proof of an hypothesis is essentially a deductive process. In verifying an hypothesis, we reason in some such fashion as this: If this hypothesis is true, then such and such consequences should follow; these consequences, however, do follow-that is, they are in accord with all the known facts bearing on the matter; therefore the hypothesis is true.

The familiar story of how Torricelli proved that the

1 Alfred Russel Wallace, Natural Selection.

air has weight, and incidentally invented the barometer, illustrates the method perfectly. It had been noticed by his master, Galileo, that water would not rise in a suction pump beyond thirty-two or thirtythree feet. Torricelli, in trying to explain why it should rise at all, hit upon the idea that it was because of the pressure of the atmosphere, the weight of the air balancing the column of water. If this were so, he reasoned, then a liquid heavier than water would rise to a less height. Mercury, for instance, which is a little more than ten times heavier than water, would rise less than one tenth as far. On inverting, in a basin of mercury, a glass tube about four feet long and hermetically sealed at one end, he found that the result was as he had conjectured. The mercury in the tube sank to about thirty inches above the level of that in the basin. His hypothesis was thus verified, and the world was benefited by the invention of a very useful instrument.

The verification of the hypothesis should be conducted, it is needless to say, with the utmost care possible. Accurate observations and rigid scrutiny of the facts used is indispensable as a preliminary, since no induction based on doubtful facts can have much validity. But even if the writer is sure of his facts, he needs to be cautioned against generalizing on too narrow a basis. He needs to be cautioned also against the assumption that a single test satisfactorily passed necessarily establishes an hypothesis. In some cases, such as the one just cited, a single test

may be sufficient; but in most cases it is not. It oftens happens that a phenomenon results from a complexity of causes, in which case a number of tests made under varying conditions will be necessary to reveal all the causes. For example, at a given altitude the application of a certain degree of heat to water in an open vessel will cause it to boil; but we cannot therefore infer that water will always boil when brought to this same temperature. At a lower altitude, it will not boil until a higher temperature is reached. In other words, pressure as well as hea must be taken into account in determining the boiling point of a fluid.

To guard against error, therefore, the tests employed in verifying an hypothesis should be as many and as varied as possible. Moreover, wherever it is convenient, experiment should be used as an aid to observation. When we can control the conditions under which a phenomenon occurs, we can the more readily determine the cause of that phenomenon.

110. Evidence. - A proposition to be argued is a matter which is in doubt; and to convert it from a matter of doubt to a matter of certainty, something must be brought forward in its support. Anything so brought forward, that is, anything used as a means of inducing belief in the truth of the proposition, is called an argument, or sometimes, as in courts of law, evidence. The whole body of arguments or evidence used to establish the truth of a proposition is called the proof.

III. Kinds of arguments. A thoroughly satisfactory classification of arguments is not easy to find. Perhaps the most convenient, on the whole, is that which groups them as follows: (a) arguments from antecedent probability, (b) arguments from sign, (c) arguments from testimony, and (d) arguments from analogy.

(a.) The argument from antecedent probability.—The argument from antecedent probability is one drawn from the relation of a cause or a necessary condition to an effect. It tries to account for the fact or circumstance in debate by bringing forward some other fact or circumstance preceding it and related to it as cause is related to effect. It infers what is likely to happen from what has happened, what is likely to be true from what is admittedly true.

The value of the argument from antecedent probability depends largely upon the clearness with which the cause and effect relation is brought out. If that relation is made unmistakable, the force of the argument is very great, since it establishes a strong presumption in favor of the proposition to be proved. Of itself, however, it is not conclusive. Thus, in a murder case, while it is very important that the accused be shown to have had a motive for committing the crime, the fact that he had a motive would not of itself prove that he committed the murder. That a thing is likely to happen is no proof that it has actually happened. The argument from antecedent probability, therefore, though a strong argument,

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