[Footnote 60: _Island Life_, p. 205.] [Footnote 61: Of course, Mr. Wallace does not believe that it is actually 200,000,000 years since the Cambrian period.] I think it must now be perfectly evident that the facts both of geology and of biology are utterly irreconcilable with the theory that the sun’s heat was derived from the condensation of its mass by gravitation; and that the mistake in regard to geological time has been committed by the physicist, and not by the geologist. The grounds upon which the geologists and the biologists found the conclusion that it is more than 20 or 30 millions of years since life began on the earth are far more certain and reliable than the grounds upon which the physicist concludes that the period must be less. The only real ground that the physicist has is that according to the theory which he holds of the origin of the sun’s heat a longer period is not possible. This might be considered good evidence were no other theory possible; but there is another theory, which accords with all the facts, and consequently has a strong presumption in its favour. PART III. _EVIDENCE IN SUPPORT OF THE THEORY FROM THE PRE-NEBULAR CONDITION OF THE UNIVERSE._ The nebular hypothesis, strictly speaking, is one simply intended to account for the origin of our solar system. “It is,” as remarks Professor A. Winchell, “primarily a genetic explanation of the phenomena of the solar system; and accessorily a co-ordination, in a common conception, of the principal phenomena in the stellar and nebular firmament, as far as human vision has been able to penetrate.”[62] The theory starts with the assumption that all the materials composing the solar system once existed in a state of extreme tenuity and diffusion, filling far more than the entire space included within the orbit of the most remote planet. It begins with this diffused nebulous mass tending slowly, under the influence of gravitation, towards a state of aggregation. Beyond this point the received nebular hypothesis does not extend. [Footnote 62: _World Life_, p. 196.]