Dialogues concerning Two New Sciences

Abstract

SCIENCEfrom protozoan cell to highest vertebrate i n the course of geologic agcs, he let 40 weeks (reduced to hours) represent geologic timesay 20 or 40 million years.For the third term in 1,he proportion he took the number of hours it was necessary to observe the embryonic development in order to detect an appreciable change, and obtained for an answer as the fourth term a number in years which vas much longer, even when the shortest lengths of geologic time were taken, than our historic period.So that it was clear there was plenty of geologic time for evolution to proceed at a pace so slow that it could not be detected within the historic period and still accomplish its perfect work.When it comes to attempts to estimate geologic time in years it seems to me that most persons must agree that they are not very satisfactory.This is particularly so with those of the physicists who have assumed as a basis for their calculatio~ls an origin for our planet, no longer looked upon with much favor i n the light of the facts which support the planetesimal hypothesis.These calculatioiis have also been largely invalidated by discoveries relating to the radio-activity of matter.Of all geologic time estimates, those based upon rate of denudation, and its correlativethe rate of deposition of stratified roclis, seem least unsatisfactory.When these lnethods are applied to precambrian time it is admitted they amount to little more than wild guesses.And yet we know that evolution was %ell on its way before the beginning of Cambrian time.Walcott has brought to light in the Canadian Rockies abundant evidence of a rich and by no means low$ organized marine fauna a t the very beginning of Cambrian time.H e and others estimate that at least 90 per cent. of the total evolution to the present had taken place before the Cambrian period.Le Conte, even before he had had the benefit of these tliscoveries, was inipressed with the high type of the Cambrian faunas.His memorable words i n this connection are: When the curtain goes up on geological history