328 The Bate of Geological Change. [July, 



frequently than that phase which we have just reviewed ; but with 

 this striking difference, that no geologist has treated it from the point 

 of view of pure geology in the same direction as Professor Phillips 

 has from a palseontological standpoint, — the geological arguments 

 are all either purely Uniformitarian, or as purely Catastrophic. 



In the second of two essays " On the Measurement of Geolo- 

 gical Time." which were published in ' Xature ' only a few weeks 

 ago,* Mr. Wallace touches on this question. He remarks f that for 

 the last 60.000 years the eccentricity of the earth's orbit has been 

 very small, and that therefore the opposite phases of precession, each 

 lasting 10,500 years, have during that time produced scarcely any 

 effect on cliinate. This state of things, however, is regarded by 

 him, as by Mr. Croll, as quite exceptional, for during nearly the 

 whole of the last three million years the opposite state of things has 

 existed, namely, a high eccentricity coupled with a change (in the 

 extra-tropical regions) every 10,500 years, from a very cold to a 

 very mild climate. Mr. Wallace therefore argues as follows: — 

 " This will necessarily have caused much migration both of plants 

 and animals, which would inevitably result in much extinction and 

 comparatively rapid modification. Allied races would be continu- 

 ally brought into competitiou, altered physical conditions would 

 induce variation, and thus we should have all the elements for 

 natural selection and the struggle for life to work upon and develop 

 new races. High eccentricity would therefore lead to a rapid change 

 of species, low eccentricity to a persistence of the same forms ; and, 

 as we are now, and have been for 60,000 years, in a period of low 

 eccentricity, the rate of change of species during that time may he 

 no measure of the rate that has generally obtained in past geological 

 epochs? 



I shall not stop to criticize Mr. Wallace's attempt to measure 

 geological time, as Mr. Dawkins has already pointed out the fallacy 

 involved in the major premiss of this argument, viz. that all climatal 

 change has depended solely on the eccentricity of the earth's orbit. I 

 It is sufficient for my present purpose to point out that Mr. Wallace 

 recognizes the principle that the rate of change of species may have 

 varied in different geological epochs. To refer the cause of its 

 variation to differences in the eccentricity of the earth's orbit is pro- 

 bably erroneous, but that error of ultimate explanation by no means 

 diminishes the importance or the stability of the fact which is thus 

 sought to be explained. 



Both Sir Charles Lyell and Mr. Wallace have attempted to 

 estimate the duration of the several geological epochs, basing their 

 calculations on a supposed rate of change in species of marine 

 mollusca. In this way Sir Charles Lyell concludes that u we may 



* February lTtli and March 3rd. t Loc. c>t., March 3rd, pp. 453 and 454. 



% See 'Nature.' March 17, p. 505. 



