Page:Popular Science Monthly Volume 9.djvu/694

666 species of animals change is tolerably uniform. The fossils of one age differ but little from those of ages immediately preceding and following it. We must go back, he says, to a period when the marine shells differ as a whole from those now existing to form one complete period. Counting back in stages measured by changes of fossils, we have four such stages in the tertiary formations above the chalk.

Lyell saw reason to believe, on evidence which we shall presently examine, that the age of ice commenced about a million of years ago. The place of this age of ice among the series of fossil-changes is easily marked, and so he concludes that each of his four periods above the chalk "would lay claim to twenty millions of years." We must allow Sir Charles to work up to his stupendous conclusion in his own words:

Eighty millions since the lower tertiary formation, one hundred and sixty millions since the formation of the coal-measures, and two hundred and forty millions since the beginning of the Cambrian period! And beyond that inconceivable antiquity lie the whole range of the primary rocks which contain no fossils.

Mr Darwin assigns to the world even a greater age. "In all probability," he says, "a far longer period than three hundred millions of years has elapsed since the latter part of the secondary period." Other geologists exceed even this estimate. Mr. Jukes, for instance, after referring to this passage, in which Mr. Darwin has given an estimate of the length of time necessary for wearing down the space between the North and South Downs, declares it is just as likely that the time which actually elapsed since the first commencement of the erosion, till it was nearly as complete as it now is, was really a hundred times greater than his estimate, "or thirty thousand millions of years!"

To any one but a professed geologist, it would almost seem as if these ideas of geological periods had been framed on the principle which guided Mr. Montague Tigg in fixing the capital of the Anglo-Bengalee Disinterested Loan and Life-insurance Company. "What," asked the secretary, "will be the paid-up capital according to the