brought out more clearly than has Mr. Wallace in this work the indefinite variety and unknown possibilities of the forces of nature. Under changed conditions, without violating at all the general uniformity of nature, other forms of organisms may be evolved which the limited conditions prevailing upon the earth will not allow, and which our limited experience can not even conceive. We have been reminded very forcibly of late that new discoveries produce many revolutionary movements within the general body of received opinions. How tremendously have the Roentgen rays and the radio-activity of radium changed our views as to the possibilities of physical forces. Moreover, as regards the stellar motions, it was held to be a matter of most obvious certainty that they were to be accounted for solely by the laws of gravitation. This position, however, has been recently questioned. The following, which Mr. Wallace has quoted in the work before us, bears testimony to a radical shifting of fundamental considerations: "I doubt whether the principal phenomena of the stellar universe are consequences of the law of gravitation at all. I have been working myself at spiral nebulæ, and have got a first approximation to an explanation—but it is electro-dynamical and not gravitational. In fact, it may be questioned whether, for bodies of such tremendous extent as the Milky Way or nebulæ, the effect which we call gravitation is given by Newton's law; just as the ordinary formulæ of electrostatic attraction break down when we consider charges moving with very great velocities" (p. 292). This statement is taken from a letter of Mr. E. T. Whittaker, Secretary to the Royal Astronomical Society, written in reply to certain questions which had been sent out by Mr. Wallace to various men of science. Now, inasmuch as such changes in the fundamental conception of the constitution of matter and the nature of physical forces have taken place, and are taking place, is it not reasonable to insist that the possibilities of unknown conditions which may obtain in unknown regions are wholly incalculable? It is extremely hazardous to state any exact limits which even present known conditions may be regarded as necessitating. The possibility of variation, of new developments, of the manifestation of newly discovered properties in connection with phenomena of exceedingly great complexity must be reckoned with.
Moreover, essential conditions so regarded might prove to be unessential, or at least capable of radical modification, if only the horizon of knowledge were lifted somewhat. And even in the world of science at present, there is much difference of opinion as to what are to
