528 Letters, Announcements, S^c. 



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. Seebohra's hypothesis. Certainly he is mistaken in the 

 inference he draws at p. 238, 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 tliat 

 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 Verte- 

 brata in this respect, I believe, too, that the distribution of 

 Plants differs materially from that of Vertebrate animals. 



W. T. Blanford. 



September 15, 1886. 



