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 various species would be able 
to adapt themselves to them without much disturbance ; 
and the result would be an epoch of exceptional stability 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 palaeolithic 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 
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, Yol. 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 
