Letters , Announcements , fyc. 527 
The Glacial epoch was comprised in the Pleistocene, with 
which it may be regarded as practically equivalent. 
Most of the birds’ bones which have been found in cave- 
breccias and other Pleistocene deposits appear to have been 
those of species that are still living at the present day; and 
there is no reason to believe that changes in birds have taken 
place more rapidly than in Mammalia, the fossil remains of 
which are far more abundant. Indeed the evidence points 
rather the other way. Now the majority of the Pleistocene 
Mammalia are living species and, so far as it is practicable to 
judge, nearly all—perhaps all truly generic forms, such as 
are adopted amongst birds by Mr. Seebohm—date from the 
Pliocene at least, whilst many are older. 
In cases such as those of Scolopax and Cursorius , where 
the differentiation suggested only extends to a few allied 
species, there is a greater possibility that Mr. Seebohm may 
be correct. Indeed some years ago I suggested a similar 
origin—the intervention of an ice-covered continent in the 
Glacial epoch—to account for the separation of certain allied 
species of migratory birds in Eastern and Western Asia; 
but the hypothesis should be used with caution. I doubt 
whether, in any case, all the species of any considerable 
genus have been differentiated since the commencement of 
the Pleistocene. 
Mr. Seebohm’s geological views are also, I think, in several 
respects open to exception. It is possible that there may 
have been half a dozen alternations of glacial and interglacial 
phases in the Pleistocene epoch (p. 226), but it is at least 
equally probable there were none ; and certainly the evidence 
existing for more than one interglacial phase is quite insuffi¬ 
cient to justify the founding of arguments upon it. Then 
what reason is there for supposing that at one portion of the 
Glacial epoch a glacier (ice-sheet?) stretched “'from the North 
Pole down the mountains of Greenland” (p. 226), and that 
in a subsequent phase of that epoch a different glacier ex¬ 
tended “ across the North Pole from the Rocky Mountains 
either to Novaya Zemlya or to the mountains of Eastern 
Siberia” ? (p. 227). Unless a change in the distribution of 
