March io, 1910] 



NATURE 



in some measure even influenced the observations cf 

 geologic phenomena well down to the close of the century, 

 and is far from obsolete to-day. 



But, logical and plausible as was this general conception 

 of earth-history, it was hung, as you have not failed to 

 notice, on the hypothesis of the genesis of the earth 

 accepted. However logical, its logical strength was only 

 that of the hypothesis on which it was hung. I say its 

 logical strength advisedly, for outside the logic of the 

 general concept there was always the appeal to the concrete 

 evidences of the geologic record. This appeal was made, 

 and was thought to be on the whole confirmatory. The 

 strata of high latitudes were found to contain relics of life 

 of tropical or subtropical types, not only in the early 

 stages, but well down toward recent times. Figs and 

 magnolias grew in Greenland as late as the Tertiary period. 

 Phenomena so striking gave deep hold to the logical scheme. 

 Phenomena not so consonant with it were easily overlooked 

 or lightly passed by, as is our wont when too much 

 impressed by what must be true. It is, however, a merit 

 of modern science that it puts that which is to the front, and 

 that which logically viust be in a secondary place ; and so, 

 during the past century, inconsonant data were gathered 

 with the consonant. Most of the inconsonant facts were of 

 the unobtrusive sort, but yet some of them were startling, 

 were seemingly incredible, were indeed long doubted, and 

 only slowly gained credence. The accumulation of this 

 inconsonant data gradually weakened the hold of the 

 general logical concept and prepared the way for a 

 reconsideration. 



Meanwhile a serious source of doubt had arisen on the 

 logical side, from the progress of physics. The older hypo- 

 theses of the origin of the earth had been framed before the 

 kinetic theory of gases was evolved. .After the kinetic view 

 was accepted, it was pointed out by Johnstone Stoney that 

 the velocities of the molecules of the outer air place a limit 

 to the volumes which planetary atmospheres may possess. 

 When the test which this suggested was applied to the 

 postulated atmospheres and voluminous gaseous states of 

 the early earth, it gave rise to grave doubt as to the 

 physical consistency of these conceptions. 



Weakness also arose in another quarter. One of the 

 main props of the gaseous or quasi-gaseous hypotheses was, 

 as already remarked, the general conviction, based on 

 dynamical grounds, that condensation from any other 

 nebulous state than the gaseous or quasi-gaseous would 

 give revolutions and rotations to the planetarv system at 

 variance with those actually possessed. A re-examination, 

 however, near the close of the century, developed grounds 

 for the conviction that a gradual gathering in of matter 

 from a scattered orbital state would give rotations and 

 revolutions quite as well in accord with the facts formerly 

 known, and seemingly even better in accord with new facts 

 recently brought to light. 



Thus toward the close of the last century there arose 

 from different quarters cogent reasons for a reconsideration 

 of the prevailing general view, and with it a recast of the 

 former forecast. Further scrutiny added new doubts to 

 those that had previously arisen, and in the end the verity 

 of the older hypotheses of genesis was challenged, and new 

 conceptions, based on orbital dynamics, in contrast to 

 gaseous dynamics, were offered in their stead. 



It is not appropriate for me to say that this challenge 

 was successful, or that the older conceptions of the earth's 

 origin are to be laid on the shelf. As an advocate of the 

 method of multiple working hypotheses, it belongs to me 

 to beg of you to save and to use, so far as you can find 

 use in them, all the hypotheses that seem to you to be 

 capable of working at all. Much less would it be appro- 

 priate for me to affirm that any form of the newer con- 

 ceptions is entitled to take the place of the older in your 

 complete confidence. The final adjudication of genetic 

 hypotheses can only come of long and patient trial by 

 searching analysis, by scrutinising logic, and by application 

 to the multitudinous phenomena which the earth, not only, 

 but the solar and stellar svstems, present. It is sufficient 

 warrant for the present review, however, that not a few of 

 the more incisive students of these things have been led 

 seriously to reconsider the foundations of the hypothesis of 

 earth-genesis that have been offered, old and new, and to 

 examine with renewed care the interpretations and infer- 

 ences that have been made to hang upon them. Whatever 



NO. 2106, VOL. 8s] 



may be your personal leanings, you will, no doubt, agree 

 that it seems less laudable now to hang prophecies of the 

 future upon hypotheses of genesis than when certain cf 

 these hypotheses received the almost universal assent of 

 those then best qualified to hold opinions respecting them. 



It does not seem to be going too far, moreover, to say 

 that, whereas we seemed to be shut up to hypotheses of 

 genesis that gave the earth a gaseo-molten state at the 

 start, it now seems, to some students at least, possible that 

 the earth inherited a quite different state from a slow- 

 growth from planetesimal or other accretions. If diverse 

 views are thus permissible, they offer alternative workin;ii 

 conceptions, and thus help to give freedom of jnterpretati.>!i 

 while they stimulate observations on the critical phenomena. 

 We may, therefore, be permitted first to review the states 

 assigned the early earth by the competitive genesis offered, 

 and then the critical phenomena that bear upon the earth's 

 future. 



Quite in contrast with the older pictures of a primitive 

 earth cooling from a gaseous state, the planetesimal h\-po- 

 thesis, which may be taken as representative of theories 

 based on concentration from a dispersed orbital state, 

 postulates a solid earth growing up slowly by accessions, 

 and becoming clothed gradually with an atmosphere and a 

 hydrosphere. Each of the fundamental parts, the earth, 

 the air, and the water, is made to grow up thus together 

 from smaller to larger volumes without necessarily attain- 

 ing at any stage a very high temperature. The early 

 sources of growth for the atmosphere and the ocean, though 

 reduced in later time, continued to serve as sources of 

 replenishment when the familiar agencies of loss came into 

 play in the later ages. Thus, far from assigning at the 

 start a vast atmospheric and oceanic supply, and assuming 

 progressive depletion of this with the progress of time, the 

 newer view starts with a minimum supply and rests on 

 means of feeding which are held to run hand in hand with 

 the sources of loss and more or less completely to com- 

 pensate them in a varying way. The question of the future 

 under this view is, therefore, not how long beyond the 

 present day will the original supply last, but rather how 

 long will the oscillating compensation of loss and supply 

 remain effective? Or, in other words, how long will the 

 past degree of equilibrium between the opposing agencies 

 keep the critical conditions within the limits required by 

 life? This question turns us quite away from any serious 

 dependence on the original states, and centres attention 

 on the geologic record and on the potency of agencies still 

 in action. Are the chief agencies which have controlled 

 life conditions for tens of millions of years past still in 

 good working order and likely to continue effective for a 

 long era yet to come, or do they show clear signs of 

 declining power portending an early failure? Let us enter 

 a little closer into the consideration of the specific factors 

 on which life depends, though time will not permit us to 

 go far. 



The pre-scientific fear that the end of life wnll come by 

 cataclvsm is not yet obsolete, nor is it theoretically im- 

 possible, but violent agencies are among the least to be 

 feared. Life might, indeed, be imagined to be in jeopardy 

 from volcanic and seismic convulsions, but they really offer 

 no serious menace to life in general, and appear never 

 to have done so in the known ages. The deadliness of 

 these boisterous catastrophes impresses itself unduly on the 

 emotions. The real peril, if peril there be, lies in the 

 deadiv unbalancing of agencies of the quiet sort. 



The conditions essential to the maintenance of the habit- 

 abilitv of the earth are many, but the more critical factors 

 either lie in the atmosphere itself or are intimately 

 associated with it. The point of keenest interest is the 

 narrowness of range to which these mobile factors are 

 confined. The several constituents of the atmosphere might 

 each or all easily be too scant or too abundant. In a 

 peculiar sense is this true of the carbon dioxide, which, 

 though one of the least, is pre-eminently the decisive con- 

 stituent of the atmosphere. A small proportion of carbon 

 dioxide is essential to plant life, and so to animal life, 

 while a large proportion would be fatal to air-breathing^ 

 animals. If the three or four hundredths of one per cent, 

 now present were lost, all life would go with it ; if it were 

 increased to a few per cent., the higher life would te 

 suppressed or radically changed ; and yet, on the one 

 hand, the theoretical sources of supply are abundant, while. 



