528 
Letters, Announcements, fyc. 
land and water took place in the interim, the ice of one cold 
cycle would probably occupy the same area as did that of the 
preceding cycle. Without some evidence of the area occupied 
by ice-sheets at different times, it is scarcely scientific to 
suggest hypothetical glaciers in order to account for hypo¬ 
thetical specific differentiations. Moreover the area of the 
ice-sheets in North America and Western Europe is fairly 
well ascertained, and does not, so far as 1 can see, support 
Mr. Seebohm’s hypothesis. Certainly he is mistaken in the 
inference he draws at p. 228, where he says, “ The shores of 
the Pacific are so much more mountainous than those of the 
Atlantic, that the ice of the Glacial periods must have extended 
much further south on the former than it did on the shore of 
the Atlantic.” In the Eastern hemisphere the statement 
that the Pacific coast is much more mountainous than the. 
Atlantic is open to question, whilst in America it has been 
ascertained that despite the mountains in the neighbourhood 
of the Pacific the ice did not extend so far south along that 
shore as it did on the Atlantic. This necessarily follows 
from the arrangement of the isothermal lines, which show the 
west coast of each continent to be the warmer, and which 
must have shown similar differences in the ice-age, as they 
are due to the earth’s rotation. 
These are, however, matters of minor importance. Upon 
the main question, I think that Mr. Seebohm over-estimates 
the effects of the Glacial epoch, and that the origin of existing 
genera and, to a great extent, of existing species of birds is 
of higher antiquity. On the other hand, I heartily agree 
with Mr. Seebohm in believing that the laws of geographical 
distribution are not the same for the whole animal kingdom; 
and in confirmation of his argument on this head I may 
mention that, as I pointed out two years ago, in an address 
to the Geological Section of the British Association, there are 
some important differences between land Mollusca and Yerte- 
brata in this respect. I believe, too, that the distribution of 
Plants differs materially from that of Vertebrate animals. 
W. T. Blanforo. 
September 15,1886. 
