454 
climate. This will necessarily have caused much migra- 
tion both of plants and animals, which would inevitably 
result in much extinction and comparatively rapid modifi- 
cation. Allied races would be continually 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 there- 
fore lead to a rapid change of species, low excentricity to 
a persistence of the same forms ; and, as we are now, and 
have been for 60,000 years, in a period of low excentricity, 
the rate 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 his- 
torical period as well as during the preceding Neolithic 
age, although slight changes of climate and of physical 
geography have undoubtedly taken place; and it would 
prove to be not so much the wsva/ly slow rate of organic 
change, as the fact of our living in the midst of an excep- 
tionally uniform climatic epoch, that has hitherto pre- 
vented us from obtaining a measure of the average duration 
of species. 
These considerations have an important bearing on our 
estimate of the duration of the glacial epoch itself, and 
on our calculation of geological time from the change of 
species since its commencement. If it terminated 70,000 
years ago, and if each 10,500 years before that date, there 
was alternately a warm period and a glacial epoch, there 
would necessarily occur aseries of northern and southern 
migrations of animals and plants, and thus deposits 
formed at times not geologically remote, might contain 
very distinct groups of animals. These might even meet 
and be confounded in the same strata, and thus lead to 
that extraordinary mixture of northern and southern 
forms which occurs in some of the more recent formations, 
like the hippopotamus and mammoth in the Norfolk 
crag and the lower brick-earths. 
Geologists seem hardly to have attached sufficient im- 
portance to the great gap that intervenes between the 
Paleolithic and Pre-historic ages.. Mr. Boyd Dawkins 
has shown, from a careful study of their mammalian 
remains, that the whole of the post-glacial river deposits 
and cave-beds of this country (148 in number) are of the 
same age, being characterised by about twenty species of 
extinct or arctic mammalia, and this was the age of 
Paleolithic man. In the Pre-historic or Neolithic age all 
these have disappeared, while the sheep, goat, dog, and 
Bos longifrons are first met with. Now, on the theory that 
this Paleolithic age was entirely post-glacial, and that 
the climate and physical geography of Britain have been 
since slowly approaching their present condition, how is 
this great gap to be accounted for? The large number of 
places where remains have been found, shows that the 
conditions requisite for preserving them very frequently 
occurred, and there must therefore have been some special 
cause which has prevented any record being left of the 
long period during which they were becoming extinct. 
Mr. J. Scott Moore* maintains that they were all pre- 
glacial, and that the gap was the glacial epoch itself. 
He adduces in corroboration the striking fact that none of 
the supposed post-glacial gravels ever rest on the boulder 
* Pre-glacial Man and Geological Chronology. Dublin, 1868. 
NARORE 
' 
| March 3, 1870 
clay, but always on an older rock, which could hardly 
have been the case in every instance were they all post- 
glacial. Again, Mr. Boyd Dawkins tells us that the 
identity of such a large proportion of the species of pre- 
glacial and post-glacial mammals “ forbids the idea of the 
existence of any gap or lacuna which would warrant the 
classification of the one as tertiary and the other as 
quaternary.” But if we admit the occurrence, during the 
last glacial epoch, not only of one or two, but of a series of 
alternate cold and warm periods, we may make the Palao- 
lithic age z¢er-glacial, and suppose it to haye occupied 
several of these alternations of climate. If we further 
place the last submergence which separated Britain from 
the Continent, during one of the later phases of extreme 
cold, when most of the extinct mammalia, as well as man, 
had migrated southwards, we shall sufficiently account for 
the great gap that intervenes between the Paleolithic and 
Pre-historic ages. 
In the “ Principles of Geology,” 1oth ed., vol. i. p. 300, 
Sir C. Lyell has given an estimate of the duration of 
geological epochs, from the proportionate change in the 
species of marine mollusca, taking as a basis a million 
years since the beginning of the glacial epoch. Of the 
marine shells then living, six per cent. have become ex- 
tinct, while at the close of the glacial epoch they were all 
of existing species, but this does not necessarily imply 
that the former are many times older than the latter. The 
glacial period itself may have been the cause of their 
extinction independently of mere time ; so that the Brid- 
lington beds, where the above-mentioned proportion of 
extinct species occurs, need not on this account be more 
than twice as old as those glacial or post-glacial drifts 
which contain only living species, or, according to our 
previous estimate, about 140,000 years. The Norfolk 
crag, which contains eleven per cent. of extinct shells, 
may be from 40,000 to 60,000 years older ; this will allow 
for two or three alternations of warm and cold periods, 
which, at a time of such high excentricity, must have been 
strongly contrasted, and have led to a correspondingly 
rapid change of species. From these considerations it 
becomes evident that the time, measured by the occurrence 
of five per cent. of extinct species of marine shells, is not 
necessarily the whole number of years which has elapsed 
since they existed, but only that number 722s the iast 
60,000 years of uniform climate and specific immobility ; 
and we may be even too lavish of time if we allow so 
much as 100,000 years for this amount of change under 
the influence of those repeated alternations of climate 
which have characterised the last three million years and 
which have probably more or less characterised all past 
geological time. If now we take this number as our 
datum instead of a million years, all Sir Charles Lyell’s 
figures will be reduced to a tenth, and will stand thus : 
the time elapsed since the beginning of the— 
Lower Miocene 2,000,000 years. 
Eocene . 6,000,000 _—,, 
Cretaceous . 10,000,000 ,, 
Einiassicissears) aie une 14,000,000 ,, 
Permian 3 is os |e ve | TG;O00/000T—s iy 
Carboniferous . . . . 18,000,000 ,, 
Devonian “. = & 3. -) 20;000;,000) 8 |; 
Silurian’ 3 ">. V2) (on is) B27600;000) 5 77, 
Cambrian =< + . . 24,000,000 ,, 
These figures will seem very small to some geologists 
who have been accustomed to speak of “millions” as 
