The late Dr. Croll, feeling the force of these astronomical conclusions, thought it possible to add sufficiently to the sun's heat to extend its rule backwards approximately 100,000,000 years by the supposition of a collision with it of another moving body of near its own size. Professor Young and others have thought that possibly the heat of the sun might have been kept up by the aid of the impact of asteroids and meteorites for a period of 30,000,000 years. Mr. Wallace obtains similar figures by estimating the time required for the deposition of the stratified rocks open to examination upon the land surface of the globe. As a result of his estimates, it would appear that 28,000,000 years is all the time required for the formation of the geological strata. From all this it is evident that geologists are much more restricted in their speculations involving time than they thought themselves to be a half-century ago. Taking as our standard the medium results attained by Wallace, we shall find it profitable to see how this time can be portioned out to the geological periods, that we may ascertain how much approximately can be left for the Glacial epoch. On all hands it is agreed that the geological periods decrease in length as they approach the present time. According to Dana's estimates,[EU] the "ratio for the Palæozoic, Mesozoic, and Cenozoic periods would be 12:3:1"--that is, Cenozoic time is but one sixteenth of the whole. This embraces the whole of the Tertiary period, during which placental mammals have been in existence, together with the post-Tertiary or Glacial period, extending down to the present time; that is, the time since the beginning of the Tertiary period and the existence of the higher animals is considerably less than two million years, even upon Mr. Wallace's basis of calculation. But if we should be compelled to accept the calculations of Sir William Thomson, Professor Tait, and Professor Newcomb, the Cenozoic period would be reduced to considerably less than one million years. It is difficult to tell how much of Cenozoic time is to be assigned to the Glacial period, since there is, in fact, no sharply drawn line between the two periods. The climax of the Glacial period represented a condition of things slowly attained by the changes of level which took place during the latter part of the Tertiary epoch. [Footnote EU: See revised edition of his Geology, p. 586.] In order to estimate the degree of credibility with which we may at the outset regard the theory of Mr. Prestwich and others, that all the phenomena of the Glacial period can be brought within the limits of thirty or forty thousand years, it is important to fix our minds upon the significance of the large numbers with which we are accustomed to multiply and divide geological quantities.[EV] [Footnote EV: See Croll's Climate and Time, chap. xx.]