Page:Popular Science Monthly Volume 48.djvu/606



NTHUSIASTIC adherents have compared the principle of natural selection with the principle of gravitation. The comparison is not warranted. In the first place the one is far from having a like cardinal value with the other as a scientific truth; and in the second place it is not the sole cause of the phenomena to be explained, as Mr. Darwin himself admitted when recognizing the inherited effects of use and disuse. Nevertheless, after making these reservations, I will for a moment adopt the comparison; because, by its aid, I shall be enabled clearly to show the nature of a widely prevalent misconception.

Let us suppose that our days were the days when Newton had lately propounded his theory, and that the newspaper reader (or as there were few such in those days, let us say "the man in the street") had been told about it. Suppose it had been explained to him that, according to Newton, bodies attract one another directly as their masses and inversely as the squares of their distances, and that the phenomena presented by the Solar System had been accounted for by him as conforming to this law. Suppose that presently the man, thus far instructed, learned that there were skeptics: Clairaut, for instance, having found that certain of the Moon's motions could not be explained as results of gravitation, and that consequently Newton's interpretation of planetary motions was untenable. Now suppose the man inferred that along with the theory of gravitation the theory of the Solar System must be abandoned; and that certain views of Copernicus, of which he had heard, and certain other views of Kepler, had been disproved. What, in such case, should we say? Evidently that the man made a profound mistake in identifying the theory of gravitation with the theory of the Solar System. We should say that there were independent reasons for accepting the Copernican system and the laws of Kepler; and that though, were the law of gravitation disproved, the pre-existing theory of the Solar System would lack that rational interpretation which the law of gravitation gave to it, yet it would remain standing on conclusive evidence.

Mr. Darwin's doctrine of natural selection and the doctrine of organic evolution are, by most people, unhesitatingly supposed to be one and the same thing. Yet between them there is a difference analogous to that between the theory of gravitation and the theory of the Solar System; and just as the theory of the Solar