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BEFORE passing to the consideration of the evidence of Evolution, it will be well to consider the nature of the evidence which can be adduced. When the details of the hypothesis are under consideration, it will be found that evidence of modification under existing influences is abundant, and that the inference that such changes have a wider scope of action is the only inference which is not supported by direct observation. The inference that such changes have taken place in the cases to which they have been extended by the hypothesis is borne out by its explaining a vast number of complicated phenomena. These phenomena are inexplicable on the other received theory (that of special creations); or they are thought to be explained by the addition of hypotheses which are not the result of inference from known facts, but which are merely known facts stated in other words, or generalizations of the facts themselves. For instance, it is said that rudimentary organs exist in conformity to a type ;

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and this is only a generalization and not an explanation of the facts. It is merely the restatement of the existence of rudimentary organs in other words.

It is often said that we are not justified in receiving an inference as true because it explains a large number of facts. Let us examine how far this is the case. Suppose an inscription were found in an unknown language, and that a single sentence occurred repeatedly in the inscription, together with numerous subordinate sentences or clauses which were different in each case, but nevertheless always predicated something of the main subject. Let us conceive that some of the subordinate sentences were made out by a knowledge of a language similar to that in which the inscription is written. Let us then suppose that an hypothesis is framed as to the meaning of the main sentence; it is quite possible that this hypothesis may be entirely erroneous, and yet the relation of the subordinate sentences which occur in conjunction with it may become apparent by virtue of the hypothesis. Imagine, for instance, that the inscription refers to an Ibis embalmed by the ancient Egyptians; and that the main sentence describes the existence of this Ibis, whilst the subordinate sentences describe its attributes and the process of embalming. Let the hypothesis be that the main sentence relates to a king, and it is quite

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possible that the whole might appear perfectly congruous when read. Here we have a clear case in which the hypothesis would not be justified by its explanation of the inscription : numerous facts, the nature and relations of the words in the inscription, would be explained by an untrue hypothesis.

Exactly such a case occurs in the hypothesis that explains the nature of light by the supposed existence of an æther. A large number of observed facts are united and explained; but we do not know that they could not be otherwise explained. If the æther be proved to exist by the accelerated periods* of Encke's comet, as Mill + has already stated, the induction is greatly strengthened; but it is not rendered certain, because it is possible that the æther is not the cause retarding the motion and so accelerating the periods of the comet. The analogies between light and sound, and the impossibility for sound to pass through a vacuum, still further substantiate the theory; but it cannot be considered proven until the existence of the æther be demonstrated.

Let us revert to our illustration, and suppose that the inscription has been interpreted by the similarity

* Encke’s comet passes through its orbit in less time in each succeeding revolution, owing to its continually diminishing tangential force and consequent gravitation towards the sun. Its retarded motion produces contraction of its orbit; and its periods are thus shortened. + Logic, vol. ü. p. 19.


of most of the words in it to those of an existing language. Imagine the inscription to have been quite incomprehensible until an hypothesis was framed that it was written in an ancient language differing from some known language in certain letters. Let us assume by hypothesis that certain letters have been replaced by others, and suppose an intelligible reading results from the application of this hypothesis. Will any man doubt its truth if the inscription is at all complicated ? Is it possible that coincidences should permit an inscription to be read in this manner? And is not the reading of the inscription sufficient proof that the real clue has been found? The answer to these questions is obvious; and a very little consideration will show that the chances against any such coincidences, even in a single sentence, are almost infinite.

Yet the only evidence of the truth of the hypothesis is its explanation of the facts of the inscription by the application of known laws. Languages are known to change in the manner supposed, although the particular language may not have been known to have changed. In this case the hypothesis is proved, because it is inconceivable that the inscription could have been read if it had not been written in the manner asserted.

Let us now compare this case with the one before us. Organic nature is a complex inscription; we




know that certain animals and plants undergo modification under changed conditions; we frame an hypothesis that other animals and plants have been similarly changed. We find that we can read the inscription by this hypothesis ; that is, numerous and complex facts are explained by it. We make no inferences, except by extending known laws and known changes to cases in which they are not known to have occurred. We know that by selecting certain horses we have bred the race-horse and the cart-horse from a common stock. We know that for every hundred or every thousand creatures born, but few survive. We see that the strongest and best-adapted creatures survive in some cases. We infer that the stock from which we modified our race-horse and our cart-horse was modified by nature, in the struggle for life and by the “ survival of the fittest," from some other less specialized form. We then discover a complete series of transitional forms in the tertiary formation, evidencing the modification of the horse by successive stages from a creature nearly allied to the tapir. An hypothesis framed in consonance with these facts explains a vast number of complex phenomena most satisfactorily. Some phenomena undoubtedly remain unexplained; but so many are explained that we are surely justified, as in the case of the inscription, in believing that the hypothesis is proven.

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