March 17, 1870 | 
NATURE 
595 
EETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
The Geological Calculus 
Mr. WALLACE’s essay, completed in NaturRxE, No. 18, brings 
to the front the question whether or no we can measure 
the Geological Past by the historical unit of years. Have we 
any basis for fixing with any certainty the date of any geological 
epoch? Mr, Wallace’s answer in the affirmative to this is 
worthy of a careful analysis, because it represents fairly the ideas 
current in the minds of many geologists. 
There are three ways by which the attempt to solve the 
problem has been made :—(1.) The slow geological changes 
which have been noted during the period of history, such as the 
modification of coast line, the silting up of estuaries, and the 
like. This method Mr. Wallace very justly discards as being 
**too minute, too limited, and too uncertain to afford the basis 
of eyen any approximate measurement” of the geological past. 
(2.) The change in organic life. This also is a unit of measure- 
ment ‘‘ which we have not yet been able to get ; for the whole 
length of the Historical Period has not produced the slightest 
perceptible change in any living thing in a state of nature.” 
Professor Huxley, in 1869, gave expression to very much the 
same view, in his Presidential Address to the Geological Society. 
This method, therefore, of approaching the problem may also 
be given up as hopeless. (3.) The excentricity of the earth’s 
orbit which Mr. Croll has used, in making his ingenious compu- 
tation of the lapse of time since the glacial period, on the hypo- 
thesis that the severity of climate at that time was due solely to 
astronomical causes, and not, as had been previously supposed, 
to changes in the physical geography of the earth. But, as Sir 
Charles Lyell argues, since the distribution of land and water 
and the course of marine currents now modify climate, they 
cannot be fairly supposed to have had no share in causing the 
severity of the glacial period. And therefore, the fact that they 
are ignored in Mr, Croll’s computation, destroys its value as 
fixing the glacial date, although there may be astronomical 
reasons for a depression of temperature at certain times in the 
northern hemisphere without the aid of any terrestrial agent. 
This, indeed, is practically admitted by Mr. Croll, when he 
reduces the date of the last glacial period from 750,000 to about 
80,000 years ago, because of the amount of sub-aerial denuda- 
tion that has taken place since that time. There are, more- 
over, two fatal objections to any estimate that can be formed 
of the amount of denudation since the glacial period. First 
of all, the denudation now going on, over any wide area, has 
not yet been ascertained with anything like accuracy, and it 
acts unequally, even in any one limited region. Secondly, 
as we do not know the original thickness of the glacial deposits, 
or the extent to which the existing valleys were excavated in pre- 
glacial times, it is impossible to estimate the amount of denu- 
dation since that period, even if we had trustworthy data from 
our own experience. This third method, therefore, of mea- 
suring geological time, is not more satisfactory than the former 
ones. 
Mr. Wallace, however, assumes in the second part of his 
essay that the vera causa of the glacial epoch was the high ex- 
centricity of the earth ; and then he proceeds to reduce Mr. 
Croll’s lowest estimate by 20,000 years, by using precisely that 
argument of observed change in physical geography which in the 
first part was discarded as “too uncertain.” From this untenable 
standpoint hangs the following chain of reasoning. 
“* Now it is most important to observe that, for the last 60,000 
years, the excentricity has been very small—for three-fourths of 
the time less than it is now. During this time the opposite phases 
of precession, each lasting 10,500 years, will have produced 
scarcely any effect on climate, which in every part of the earth 
will have been xearly uniform for that long period. But this is 
quite an exceptional state of things ; for the curve of excentricity 
shows us that, during almost the whole of the last three million 
years, the excentricity has been high—almost always twice, and 
sometimes three and four times as much as it is now. If, there- 
fore, Mr. Croll’s theory be correct, there will have been a change 
each 10,500 years during this vast period (in all the extra-tropical 
regions at least) from a very cold to a very mild climate. ‘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 con- 
tinually brought into competition, 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 excentricity would therefore lead to a 
rapid change of species ; low excentricity to a persistence of the 
same forms ; and, as weare nowand have been for 60,000 years, 
in a period of low excentricity, the vate of change of species during 
that time may be no measure of the rate that has generally obtained 
in past geological epochs. ‘Thus we should have explained the 
extraordinary persistence of organic forms during the historical 
period, as well as during the preceding Neolithic age, although 
slight changes of climate and of physical geograghy have un- 
doubtedly taken place ; and it would prove to be not so much 
the zszzadly slow rate of organic change, as the fact of our living 
in the midst of an exceptionally uniform climatic epoch, that has 
hitherto prevented us from obtaining a measure of the average 
duration of species.” 
The major premiss latent in this argument is, that all climatal 
change from the glacial epoch to the present day has de- 
pended solely on the excentricity of the earth’s orbit, a proposi- 
tion which Mr. Wallace himself would be the last to endorse. 
If it be admitted that the alteration of a marine current 
here, or the elevation of a sea-bed there, be factors in climatal 
change, the estimate of 60,000 years in which they are not 
reckoned is without value. The study of the mammalia, 
of historic, pre-historic, and post-glacial times does not 
warrant the conclusion that the persistence of organic forms 
was ‘‘extraordinary,”’ nor the recognition of an ‘‘ exceptionally 
uniform climatic epoch.” The mammals have exhibited on the 
whole a steady diminution z7 size from the post-glacial to the 
present day, owing probably to the fact that they have been 
worried off their feeding and hunting grounds by man. The non- 
development of new species during that time may be ascribed to 
its short duration as compared with past geological epochs, 
rather than to exceptional conditions of life caused by exceptional 
excentricity. The deposits in Britain since the glacial epoch are 
a mere surface film compared with those of previous geological 
periods. The steady northern retreat of the reindeer during 
historic times, taken in conjunction with its pre-historic range, 
testifies to a gradual increase of temperature in central and 
northern Europe, to say nothing of the historical evidence of 
the former severity of winter in Gaul and Italy from Czesar’s 
time to the present. 
There is another point which ought not to be omitted. Mr. 
Scott Moore is quoted as maintaining that the group of mam- 
malia commonly called “ Quaternary” is pre-glacial because of 
the “striking fact, that none of the supposed pre-glacial gravels 
ever rest on the boulder clay, but always on an older rock, which 
could hardly have been the case in every instance were they all 
post-glacial.” So far from this being true, the famous Bedford 
section proves, as Mr. Wyatt showed in 1860, that the mam- 
maliferous and flint-implement producing gravels are post-glacial 
without the possibility of a doubt. On the Norfolk coast 
mammaliferous gravel overlies the boulder clay in certain 
places. The mammalia found in making the Ipswich tunnel 
were derived from a river deposit, clearly of later date than the 
boulder clay of the district. Very many other cases might be 
quoted that would show that this sweeping generalisation is 
without any foundation in fact. 
But can we measure geological time by the lapse of years? 
If so, we shall have solved a problem infinitely harder than that 
which has foiled the archeologists. Can they fix the date, say 
of the introduction of iron into Europe, or of the dawn of the 
age of bronze or of stone? No man would venture to answer 
yes. Modern historians are becoming more and more alive to 
the worthlessness of the so-called chronology of the Assyrian 
kings and of the Manethonian dynasties. If, then, we are igno- 
rant of the date of any one of these events, which are, comparatively 
speaking, of yesterday, and we can simply tell that one suc- 
ceeded another in a definite order, how can we reasonably expect 
to fix the date of any one period of the geological past? ‘The 
attempt can only be made by forsaking those laws of rigid in- 
duction by which geology has become a science—by the as- 
sumption of a premiss which we have no right to assume. The 
strict interpretation of geological phenomena only warrants our 
saying that, one event, say the deposition of the chalk, took 
place in Europe after another,—the deposition of the Neo- 
comian strata,—how much after none can tell. In other words, 
the geological “ when” merely implies before and after, while in 
