502 



NA TURE 



[September 24, 1S96 



Herbert Spencer, in his article on " Illogical Geology " in the 

 Universal Revieiv for July 1859 (reprinted in his " Essays," 

 1868, vol. i., pp. 324-376), uses these words : "Only the last 

 chapter of the earth's history has come down to us. The many 

 previous chapters, stretching back to a time immeasurably 

 remote, have been burnt, and with them all the records of life 

 we may presume they contained." Indeed, so brief and un- 

 important does Herbert Spencer consider this last chapter to have 

 been that he is puzzled to account for "such evidences of pro- 

 gression as exist " ; and finally concludes that they are of no 

 significance in relation to the doctrine of evolution, -but probably 

 represent the succession of forms by which a newly upheaved 

 land would be peopled. He argues that the earliest immigrants 

 would be the lower forms of animal and vegetable life, and that 

 these would be followed by an irregular succession of higher and 

 higher forms, which " would thus simulate the succession pre- 

 sented by our own sedimentary series." 



We see, then, what these three great writers on evolution 

 thought on this subject : they were all convinced that the time 

 during which the geologists concluded that the fossiliferous 

 rocks had been formed was utterly insufficient to account for 

 organic evohuion. 



Our object to-day is first to consider the objections raised by 

 physicists against the time demanded by the geologist, and still 

 more against its multiplication by the student of organic evolu- 

 tion ; secondly, to inquire whether the present state of pali-eonto- 

 logical and zoological knowledge increases or diminishes the 

 weight of the threefold opinion quoted above — an opinion formed 

 on far more slender evidence than that which is now available. 

 And if we find this opinion sustained, it must be considered to 

 have a very important bearing upon the controversy. 



The arguments of the j^hysicists are three : — 



F'irst, the argument from the observed secular change in the 

 length of the day the most important element of which is due to 

 tidal retardation. It has been known for a very long time that 

 the tides are slowly increasing the length of our day. Huxley 

 explains the reason with his usual lucidity : " That this must be 

 so is obvious, if one considers, roughly, that the tides result from 

 the pull which the sun and the moon exert upon the sea, causing 

 it to act as a sort of break upon the solid earth." (Anniv. 

 Address to Geol. Soc. , 1869.) 



A liquid earth takes a shape which follows from its rate of 

 revolution, and from which, therefore, its rate of revolution can 

 be calculated. 



The liquid earth consolidated in the form it last assumed, and 

 this shape has persisted until now, and informs us of the rate of 

 revolution at the time of consolidation. Comparing this with 

 the present rate, and knowing the amount of lengthening in a 

 given time due to tidal friction, we can calculate the date of 

 consolidation as certainly less than 1000 million years ago. 



Thisargument is fallacious, as many mathematicians have shown. 

 The present shape tells us nothing of the length of the day at the 

 date of consolidation ; for the earth, even when solid, will alter 

 its form when exposed for a long time to the action of great 

 forces. As Prof. Perry said in a letter to Prof. Tait (Nature, 

 January 3, 1S95) '■ " I know that solid rock is not like cobbler's 

 wax, but 1000 million years is a very long time, and the forces 

 are great." Furthermore, we know that the earth is always 

 altering its shape, and that whole coast-lines are slowly rising or 

 falling, and that this has been true, at any rate, during the 

 formation of the stratified rocks. 



This argument is dead and gone. We are, indeed, tempted 

 to wonder that the physicist, who was looking about for argu- 

 ments by which to revise what he conceived to be the hasty 

 conclusions of the geologist as to the age of the earth, should 

 have exposed himself to such an obvious retort in basing his 

 own conclusions as to its age on the assumption that the earth, 

 which we know to be always changing in shape, has been unable 

 to alter its equatorial radius by a few miles under the action of 

 tremendous forces constantly tending to alter it, and having 

 1000 million years in which to do the work. 



With this flaw in the case it is hardly necessary to insist on 

 our great uncertainty as to the rate at which the tides are 

 lengthening the day. 



The spectacle presented by the geologist and biologist, deeply 

 shocked at Lord Kelvin's extreme uniformitarianism in the 

 domain of astronomy and cosmic physics, is altogether too com- 

 forting to be passed by without remark ; but in thus indulging 



NO. 1404, VOL. 54] 



in a friendly tu qiioque, I am quite sure that I am speaking for 

 every member of this Section in saying that we are in no way 

 behind the members of Section A in our pride and admiration at 

 the noble work which he has done for science, and we are glad 

 to take this opportunity of congratulating him on the half-cen- 

 tury of work and teaching— both equally fruitful— which has 

 reached its completion in the present year. 



The second argument is based upon the cooling of the earth, 

 and this is the one brought forward and explained by Lord 

 Salisbury in his Presidential Address. It has been the argument 

 on which perhaps the chief reliance has been placed, and of 

 which the data — so it was believed — were the least open to 

 doubt. 



On the Sunday during the meeting of the British Association 

 at Leeds (1890), I went for a walk with Prof. Perry, and asked 

 him to explain the physical reasons for limiting the age of the 

 earth to a period which the students of other sciences considered 

 to be very inadequate. He gave me an account of the data on 

 which Lord Kelvin relied in constructing this second argument, 

 and expressed the strong opinion that they were perfectly sound, 

 while, as for the mathematics, it might be taken for granted, he 

 said, that they were entirely correct. He did not attach much 

 weight to the other arguments, which he regarded as merely offer- 

 ing support to the .second. 



This little piece of personal history is of interest, inasmuch 

 as Prof. Perry has now provided us with a satisfactory answer to 

 the line of reasoning which so fully satisfied him in 1890. And 

 he was led to a critical examination of the subject by the attitude 

 taken up by Lord Salisbury in 1894. Prof. Perry was not 

 present at the meeting, but when he read the President's address, 

 and saw how other conclusions were ruled out of court, how the 

 only theory of evolution which commands anything approaching 

 universal assent was set on one side because of certain assump- 

 tions as to the way in which the earth was believed to have 

 cooled, he was seized with a desire to sift these assumptions, and 

 to inquire whether they would bear the weight of such iar-reaching 

 conclusions. Before giving the results of his examination, it is 

 necessary to give a brief account of the argument on which so 

 much has been built. 



Lord Kelvin assumed that the earth is a homogeneous mass of 

 rock similar to that with which we are familiar on the surface. 

 Assuming, further, that the temperature increases, on the average 

 1° F.for every 50 feet of depth near the surface everywhere, he con- 

 cluded that the earth would have occupied not less than twenty, 

 nor more than four hundred, million years in reaching its present 

 condition from the time when it first began to consolidate and 

 possessed a uniform temperature of 7000° F. 



If, in the statement of the argument, we substitute for the 

 assumption of a homogeneous earth an earth which conducts 

 heat better internally than it does towards the surface. Prof. 

 Perry, whose calculations have been verified by Mr. O. Heaviside, 

 finds that the time of cooling has to be lengthened to an extent 

 which dejiends upon the value assigned to the internal conduct- 

 ing power. If, for instance, we assume that the deeper part of 

 the earth conducts ten times as well as the outer part, Lord 

 Kelvin's age would require to be multiplied by fifty-six. Even 

 if the conductivity be the same throughout, the increase of den- 

 sity in the deeper part, by augmenting the capacity for heat of 

 unit volume, implies a longer age than that ccinceded by Lord 

 Kelvin. If the interior of the earth be fluid or contain fluid in 

 a honeycomb structure, the rate at which heat can travel would 

 be immensely increased by convection currents, and the age 

 would have to be correspondingly lengthened. If, furthermore, 

 such conditions, although not obtaining now, did obtain in past 

 times, they will have operated in the same direction. 



Prof. Tait, in his letter to Prof. Perry (published in Nature 

 of January 3, 1895), takes the entirely indefensible position that 

 the latter is bound to prove the higher internal conductivity. 

 The obligation is all on the other side, and rests with those 

 who have pressed their conclusions hard and carried them 

 far. These conclusions have been, as Darwin found them, one 

 of our "sorest troubles" ; but when it is admitted that there is 

 just as much to be said for another set of assumptions leading 

 to entirely different conclusions, our troubles are at an end, and 

 we cease to be terrified by an array of symbols, however un- 

 intelligible tons. It would seem that Prof Tait, without, as far 

 as I can learn, publishing any independent calculations of the 

 age of the earth, has lent the weight of his authority to a period 

 of ten million years, or half of Lord Kelvin's minimum. But 



