CHAP. X.] THE RATE OF ORGANIC CHANGE. 



225 



decrease during all known geological time. But when the 

 glacial epoch ended, 72,000 years ago, the excentricity was 

 about double its present amount ; it then rapidly decreased 

 till, at 60,000 years back, it was very little greater than it is 

 now, and since then it has been uniformly small. It follows 

 that, for about 60,000 years before our time, the mutations 

 of climate every 10,500 years have been comparatively unim- 

 portant, and that the temperate zones have enjoyed an excep- 

 tional stability of climate. During this time those powerful 

 causes of organic change which depend on considerable changes 

 of climate and the consequent modifications, migrations, 

 and extinctions of species, will not have been at work; the 

 slight changes that did occur would probably be so slow 

 and so little marked that the variou.s species would be able 

 to adapt themselves to them without much disturbance ; 

 and the result would be an epoch of exceptional stahility of 

 species. 



But it is from this very period of exceptional stability that we 

 obtain our only scale for measuring the rate of organic change. 

 It includes not only the historical period, but that of the Swiss 

 Lake dwellings, the Danish shell-mounds, our peat-bogs, our 

 sunken forests, and many of our superficial alluvial deposits — 

 the whole in fact, of the iron, bronze, and neolithic ages. Even 

 some portion of the palseolithic age, and of the more recent 

 gravels and cave-earths may come into the same general period 

 if they were formed when the glacial epoch was passing away. 

 Now throughout all these ages we find no indication of change 

 of species, and but little, comparatively, of migration. We thus 

 get an erroneous idea of the permanence and stability of specific 

 forms, due to the period immediately antecedent to our own 

 being a period of exceptional permanence and stability as regards 

 climatic and geographical conditions.^ 



1 This view was, I believe, first put forth by myself in a paper read 

 before the Geological Section of the British Association in 1869, and 

 subsequently in an article in Nature, Vol. I. p. 454. It was also stated 

 by Mr. S. B. K. Skertchley in his Physical System of the Universe, p. 363 

 (1878) ; but we both founded it on what I now consider the erroneous 

 doctrine that actual glacial epochs recurred each 10,500 years during 

 periods of high excentricity, 



Q 



