June. 1930 



E \' O L U T I O N 



Page thirteen 



I^EW BOOKS 



THE EARTH— ITS ORIGIN, HIS- 

 TORY. AND PHYSICAL CONSTI- 

 TUTION by Harold Jeffreys. 2nd 

 Ed. New York. The Macmillan Co. 

 and The Cambridge Press, $6.50. 

 399 pages. 



There exist today only two widely 

 accepted theories of the origin of 

 the solar system, and hence of our 

 earth — the Planetesimal Hypothesis 

 of the late Dr. T. C. Chamberlin and 

 Prof. F. R. Moulton, both of the Uni- 

 versity of Chicago; and the Tidal 

 Theory of Sir J. H. Jeans and Dr. 

 Harold Jeffreys. In the past it has 

 seemed to many scientists on this 

 side of the Atlantic that these two 

 great English thinkers had ignored 

 unduly the researches of Professors 

 Chamberlin and Moulton, although it 

 was considered that the Tidal Theory 

 was largely an outgrowth of the theo- 

 ries developed by the Chicago scien- 

 tists. 



Fortunately, in this new and revised 

 edition of his great work, "The Earth," 

 Jeffreys has removed this objection. 

 In an appendix, he considers the 

 Planetesimal Hypothesis fully and 

 fairly, referring to it as "the parent 

 of the Tidal Theory," though adverse- 

 ly comparing it with Jeans' hypoth- 

 esis. Which, he remarks, "bears a 

 close resemblance to mine, the agree- 

 ment becoming striking since the two 

 methods of attack are quite different." 

 It thus becomes apparent that the 

 Tidal Theory is no isolated hypothesis, 

 but is a legitimate outgrowth of and 

 advance upon the Planetesimal Hy- 

 pothesis. Just as this, in a sense, 

 grew out of a critical examination of 

 Laplace's now discredited "Rotation- 

 al" Nebular Hypothesis. In both of 

 the modern theories it is accepted as 

 substantially established that the sun 

 was broken up by a passing star, 

 several times more massive than it- 

 self. There is, as Jeffreys states, a 

 general resemblance between the 

 modes of formation and rupture of 

 the filament or filaments, ejected 

 from the sun, in the two theories. 

 It is with the Chamberlin-Moulton 

 "planetesimals" that the break be- 

 ♦ tween the two hypotheses occurs, and 

 with the "planetesimal" contention 

 that the planets grew gradually larger 

 by accretion of these meteoritic 

 bodies. 



On the problem of the origin of 

 the moon, Jeffreys has made certain 

 changes since the first edition of his 

 book in 1926. It was then assumed 



that the sun at the time of the as- 

 sumed disruption was greatly distend- 

 ed — almost filling the orbit of Mer- 

 cury — hence below the density re- 

 quired. If the primitive sun had 

 nearly its present density, tidal dis- 

 ruption of the liquid earth was poss- 

 ible, and the moon could have been 

 thrown off as a separate body by the 

 action of the solar tides — as suggest- 

 ed, in 1S79. by Sir George Darwin. 

 This is more nearly in harmony with 

 the now known data than any altern- 

 ative theory — e. g., that the moon 

 was an independent planet, subse- 

 quently "captured" by the earth. 

 Jeffreys has, in this second edition, 

 come to accept it as highly probable 

 that, as was first suggested by Os- 

 mond Fisher, the Pacific Ocean fills 

 "the scar left when the moon was 

 formed." 



The various methoods of estimating 

 the age of the earth are "discussed 

 by Jeffreys. His general conclusion 

 is that by far the most satisfactory 

 data are to be derived from ascertain- 

 ing the ratio of the quantities of ur- 

 anium and lead in any series of ig- 

 neous rocks, the rate of degeneration 

 of uranium to lead being now known 

 to follow what appears to be an un- 

 varying law. Radium occurs in nature 

 only in the presence of uranium, 

 which itself never occurs without 

 radium. Knowing the rate of break- 

 up of uranium into its final product, 

 lead, it is a simple matter to find the 

 time since the formation of any rock 

 if we know the amounts of uranium 

 and of lead in any given specimen. 

 Jeffreys concludes that the lower limit 

 to the age of the earth obtained by 

 studying individual minerals, and the 

 upper limit given by the constitution 

 of the crust as a whole, are sufficient- 

 ly close together to fix the age as 

 roughly two thousand million years. 



Very interesting and convenient is 

 Dr Jeffreys' summation and critical 

 examination of current theories of the 

 causes of climate variation and the 

 causes of glacial epochs. "We are 

 now in a glacial period, although not 

 at its height." Several factors ad- 

 mittedly are involved, but it appears 

 that Brooks' theory is most probable, 

 that land submersion, with conse- 

 quent extension of oceanic conditions, 

 accounts quantitatively for the epochs 

 of mild climate, while general eleva- 

 tion of the land tends to the produc- 

 tion of extensive ice sheets. When 

 we add to this the important contribu- 

 tion of F. Kerner-Marilaun, in which 

 he shows that a rise of temperature 

 of two degrees would clear the Artie 

 Ocean completely of Ice, thus abol- 

 ishing the effects of floating ice and 

 cold currents, both long spells of mild 



climate and the much shorter periods 

 of glaciation are largely, if not ade- 

 quately, explained. 



Dr. Jeffreys brings against Wegen- 

 er's theory of floating continents what 

 appear to be devastating criticisms. 

 But he finds no facts regarding 

 ancient land bridges between the Old 

 and the New World which are incom- 

 patible with the results of his own 

 researches, though he does not active- 

 ly advocate the reality of such sub- 

 merged land connections, for the 

 reason that he is "not yet convinced 

 of the cogency of the palentological 

 evidence." 



There follows what I cannot but 

 regard as a decidely surprising state- 

 ment; namely, the conclusion that 

 the species concerned are plants 

 and low animals, and it seems 

 far from impossible that spores, 

 seeds, or eggs could have drifted 

 across (from continent to contin- 

 ent) on floating refuse. 

 But we know that early in the 

 Eocene — Old World mammah (not 

 "low animals") appeared as immi- 

 grants in North America, while early 

 in the succeeding Oligocene epoch, a 

 still more marked invasion occurred, 

 although the time was of short dura- 

 tion. A third extensive migration of 

 higher mammals took place in the Mio- 

 cene, and a fourth during the Pliocene. 

 Inasmuch as there is no paleontolo- 

 gist living today who doubts that the 

 various species of rhinoceros (to give 

 only one example) which appeared in 

 North America during Miocene times 

 were of Old World origin, one wonders 

 just what Jeffreys means by suggest- 

 ing that only "low forms" that might 

 have drifted across the ocean "on 

 floating refuse" have to be accounted 

 for. 



But Dr. Jeffreys does not pretend 

 to be a palaeontologist: he may rest 

 well content with the distinction of 

 being one of the world's foremost geo- 

 physicists, and the author of what 

 many experts regard as one of the best 

 works that has so far appeared in 

 the English language on the origin, 

 history and physical constitution of the 

 earth. MAYNARD SHIPLEY 



RECEIVED FOR RETIEW 



DUST TO LIFE: Burton Peter Thorn 

 THE UNIVERSE AROUND US: Sir 



James Jeans. 

 THE STORY OF EVOLUTION: Benj. 



C. Griienberg 

 HUMAN HISTORY: G. Elliot Smith 

 NEW VIEWS OF EVOLUTION: G. P. 



Conger 

 BIOLOGICAL BASIS OF HUMAN 

 NATURE: H. S. Jennings 

 THE CHILD'S HEREDITY: Paul 



PopenoG 



