Charles Callaway — Migration of Species. 
445 
Y. — The Migration of Species as related to the Cobrelation 
of Gteologioal Formations. 
By Charles Callaway, M.A., B.Sc., F.G.S. 
T HE topic of this paper is suggested by a somewbat extended 
practical study of the fossils of Nortli America. A comparison 
of these forms with our European faunas will, I tbink, tbrow some 
light upon migration, and upon the correlation of strata. 
When comparing Western with eastern formations, we are thrown 
back chiefly upon the testimony of fossils. Mineral resemblance, such 
as is observed between the Wenlock Limestone and Shale in the east, 
and the Niagara Limestone and Shale in the west, is of little value 
at great distances. The Wenlock Limestone, for instance, passes 
into an arenaceous deposit towards Wales, and the Niagara Lime- 
stone thins out towards the east in the State of New York ; so that 
the continuity of the two formations is an extreme improbability. 
Stratigraphical position, the third fest of contemporaneity, is also of 
little Service, and is itself dependent. upon fossil evidence. We 
cannot, for example, justly argue that the Niagara group of New 
York represents our Wenlock on the ground of its stratigraphical 
position, unless we admit that the formations overlying and under- 
lying it correspond respectively to our Ludlow and May Hill 
Sandstone. But we can prove that correspondence only by strati- 
graphical position, wliich would land us in absurdity ; or by fossils. 
The uncertainty of mere stratigraphical evidence is well illustrated 
by the Devonian and Old Eed Sandstone of South-westem Britain ; 
and, if this method of proof is so dubious in the case of formations 
separated by the Bristol Channel, it is obviously much more un- 
trustworthy when applied to groups on opposite sides of the At- 
lantic Ocean. 
It has been maintained that identity of species, so far from proving 
contemporaneity, is an evidence of non-contemporaneity. If, for ex- 
ample, a species migrates from Britain to America, it is clear that 
the beds containing it in America are newer than the strata which 
contain it in Britain. This is undoubtedly true if we use the word 
“ contemporaneous ” in a strictly historical sense. It is evident that 
the whole of the formation in America characterized by a certain 
fauna cannot be contemporaneous with the whole of the series in 
Britain containing the same fauna; the former will be later than 
the latter by the time occupied in the migration of the fauna a 
distance of 3000 or 4000 miles. This is on the supposition that the 
migration Was from east to west. It is possible, however, that the 
fauna may have originated in some intermediate centre, and spread 
both east and west ; in which case eastern and westem formations, 
characterized by the same (or a similar) fauna, may be strictly con- 
temporaneous. Even on the extreme supposition of an east and 
west migration, I shall attempt to show that the time occupied in 
the migration is unimportant in comparison with the duration of 
geological epochs. 
It must be remembered that, for purposes of correlation, the group 
