The sixth hypothesis is that to which most credence is now given. It is that of evolution of energy by condensation of volume. Professor Proctor ("The Sun as a Perpetual Machine") says, "In company with this great mystery of seeming waste comes the yet more difficult problem, how to explain the apparent continuance of solar light and heat during millions of years. We know from the results of geological research that the earth has been exposed to the action of the solar rays with their present activity during at least a hundred million years. Yet it is difficult to see how, on any hypothesis of the generation of solar heat, or by combining together all possible modes of heat generation, a supply for more than twenty millions of years in the past and a possible supply for as long a period in the future can be accounted for." Of these vast periods of terrestrial existence in the past we quote the following from a recent publication: "Professor C. D. Wolcott expresses the opinion that geologic time is not to be measured by hundreds of years, but simply by tens of millions. This is widely different from the conclusion arrived at by Sir Charles Lyell, who, basing his estimate on modifications of certain specimens of marine life, assigned 240,000,000 years as the required geological period; Darwin claimed 200,000,000 years; Crowell, about 72,000,000; Geike, from 73,000,000 upward; McGee, Upham, and other recent authorities claim from 100,000,000 up to 680,000,000." Helmholtz ("On the Origin of the Planetary System") says, "It is probable rather that a great part of this heat, which was produced by condensation, began to radiate into space before this condensation was complete. But the heat which the sun could have previously developed by its condensation would have been sufficient to cover its present expenditure for not less than 22,000,000 of years of the past.... We may therefore assume with great probability that the sun will still continue in its condensation, even if it only attained the density of the earth, though it will probably become far denser in its interior, owing to its far greater pressure; this would develop fresh quantities of heat, which would be sufficient to maintain for an additional 17,000,000 of years the same intensity of sunshine as that which is now the source of all terrestrial life." Of this process of condensation Professor Ball, in his recent work, "In the High Heavens," says, "It goes without saying that the welfare of the human race is necessarily connected with the continuance of the sun's beneficent action. We have indeed shown that the few other direct or indirect sources of heat which might conceivably be relied upon are in the very nature of things devoid of necessary permanence. It becomes, therefore, of the utmost interest to inquire whether the sun's heat can be calculated on indefinitely. Here is indeed a subject which is literally of the most vital importance, so far as organic life is concerned.